I don't know what degree of 'notability' you're looking for in criticism of how harshly King portrayed Smith in his book, but certainly there has been public criticism.
In a Cracked article entitled 6 Famous Works of Art You Didn't Know Were Vicious Insults, Stephen King is described as passive-aggressive and living in permanent rage at Smith. To be fair, this article itself is also pretty rude about Smith:
Stephen King Gets Endless Revenge on the Driver Who Almost Killed Him
Smith -- a drug user with 11 prior driving offenses -- was rushing to get some "Marzes bars" from the store. Yes, one of the most prolific authors in human history was almost killed because some mulleted gomer with a gun-inspired dog had a hankering for an endangered candy bar. Smith thought he'd hit a deer at first, but then he noticed a pair of bloodied glasses on his passenger seat and summoned up all his brainpower to realize that he'd never seen a deer at the optometrist's office.
[...] King's first act of revenge was to buy the minivan that almost killed him for 1,500 dollars so he could beat the shit out of it with a sledgehammer, because that's the sort of therapy you can have when you're rich.
Then things got creepier. [...] Smashing his tormentor's van with a sledgehammer and seeing Smith's physical and mental departure from this plane of existence wasn't enough for King, who had to make sure Smith lived on in infamy. His Dark Tower series is like 11 Bibles long and difficult to explain, but in it you'll find a character who has a Rottweiler named Bullet, drives a blue minivan, and is generally an all-around mouth-breather. He's an irresponsible, mentally deficient drug addict, and his name is -- cue gasp of surprise -- Bryan Smith. Smith is looking to score some drugs when he hops in his van, and almost strikes a fictionalized King, who's saved by the sacrifice of hero Jake Chambers.
[caption on image of Stephen King wearing a manic grin] "Don't forget to mention that he has a small penis. Like, just the tiniest thing you ever did see."
King's passive-aggressive jab was written years after Smith's death and served as the ultimate cherry on top of his rage sundae. Thanks to King's ceaseless anger, Bryan Smith is immortalized as a drug-addled asshat who damn near kills a beloved character, because in horror, even death can't spare you from revenge.
In her blog read-through of the Dark Tower series on the Tor Books website, reviewer Suzanne Johnson opines that King's portrayal of Smith is so insulting that it might have been grounds for libel charges if Smith were still alive:
How much of this Bryan Smith stuff is true? I found myself wondering this throughout the whole chapter. I know he had a long string of traffic violations and was charged with "driving to endanger". He claimed he was distracted by his dogs, and died just over a year after the accident from a painkiller overdose. Guess I'm just conscious of the litigious nature of society, and Bryan Smith the character is definitely portrayed as simple and stupid and so utterly irresponsible that it seems like prime defamation of character charges. Maybe if one is dead one's character can't be defamed? Anyway, he compares Smith's intelligence to that of Sheemie, who "could be Bryan Smith's litter-twin". Which, now that I think about, is quite an insult to Sheemie.
[...] Again, we're hit with lots of comments about the stupidity of the hapless Bryan Smith, "the sort of person who cannot fix the radio without looking at it". Then again, if you're writing about the careless, thoughtless dude who almost killed you, turning him into a three-dimensional character might not be possible.
I also have to wonder if there was some legal maneuvering that went on behind the scenes so King could portray Smith as such an idiot without fear of litigation.
Finally, although I think this somewhat misses the point of your question, there were also some fairly prominent claims that Smith was treated unfairly in real life due to King's fame and public regard. This article from the Chicago Tribune - in 1999, before Smith's death and subsequent immortalisation in King's writing - summarises these claims, quoting Smith's lawyer, other people from Maine, and the hard-to-pin-down 'public opinion':
The klieg-light attention has come as a surprise to a lot of peripheral figures in the case, but none more than Smith. Since he was indicted Sept. 30 on the felony charge of aggravated assault, which could carry a prison term of 10 years, Smith has charged that he is being treated harshly "just because it's Stephen King." And last week, Smith's lawyer, John Jenness of Paris, Maine, filed a motion requesting a change of venue for the trial, arguing that the aura of fame surrounding King in Oxford County has made it impossible for his client to get a fair trial.
The Oxford County assistant district attorney, Joseph O'Connor, says the charges are harsh because Smith's driving was extremely irresponsible. With jury selection not due to start until next spring, he said, it's too early to decide whether Oxford County jurors are biased. In any case, he said, it would be difficult to find a place where King was not well-known.
"Realistically, where are you going to change the venue to?" he asked. "Timbuktu?"
Sympathy poured into Bangor from around the world during the weeks after the accident. But since then, as information about Smith and the incident has come out, people in King's hometown have had time to fine-tune their opinions--in sometimes surprising ways. Scott Bryson, 23, said he and his friends were surprised by a newspaper report that Smith turned around "for three or four seconds." After he saw that, he agreed with King's position that the state should take away Smith's license.
But Bill Watson, 30, who cooks in a downtown restaurant, said King's request had turned his sympathy into exasperation. He sees Smith, who has a long series of traffic violations, as a "little guy" who has "paid (his) penance" and should be allowed to put his life back together.
"I think Steve has gone overboard," Watson said. "He came up because he wanted to get away from the big city, but what he's doing is big city."
Those who have tried to gauge public opinion in the wake of the assault charge have been surprised at how suspicious Mainers are of favoritism. Of the 50 letters the Bangor Daily News received on the subject after the indictment, four out of every five thought Smith was being treated unfairly, said Bruce Kyle, the paper's assistant editorial page editor. And when Charles Horne, news director of the Bangor talk radio station WVOM, opened up phone lines on the subject, he was "a bit surprised" at how wary Mainers were that King would be favored.
"There was no question that the tide of opinion was against the charge," Horne said. "What a lot of people were saying was Bryan Smith made the mistake of hitting a celebrity."
Some observers said they weren't surprised to see Mainers react this way, and chalked it up to the state's deep-running populism--what one lawyer termed the "underdog effect." Judy Potter, who teaches trial law at the University of Maine, said she wouldn't be surprised if Mainers "bent over backwards to make sure they weren't" treating Smith unfairly.