From the moment Curley sees Lennie, he develops a strong dislike towards him because Lennie is physically bigger than Curley. I've always wondered if there were other reasons for Curley's hatred of Lennie, and so I thought Curley might have always suspected that Lennie was cognitively disabled, and that may have contributed to his strong dislike for Lennie. Is there any textual evidence to support this?
I just read the entire book, and there is no explicit evidence of Curley describing Lennie as cognitively challenged. However, the fact that Lennie is cognitively challenged seems to be understood by most of the characters, e.g. Curley's wife (who is never named in the story), who tells Lennie that "I think you're nuts" on pg. 44. It seems likely that Curley would also be aware of this.
There are also times when Curley uses Lennie's disability to abuse him. When Curley and Lennie first meet on page 13, Curley bullies Lennie about not speaking to him:
Lennie was looking helplessly to George for instruction.
"An' you won't let the big guy talk, is that it?"
"He can talk if he wants to tell you anything." He nodded slightly to Lennie.
"We jus' come in," said Lennie softly.
There is also plenty of evidence that Curley is insecure: about his height, about his wife, and about his fighting ability. Lennie and Curley get into a fight, and Lennie beats Curley pretty decisively. I think that it would add insult to injury for Curley to know that not only was he defeated, but that he was defeated by someone who he viewed as inferior and easily bullied.
It's clear that Lennie's disability impacted the way Curley thought about and treated Lennie.
That said, there isn't any evidence of Curley being prejudiced towards people with disabilities as a class of people. I couldn't find any examples of Curley, for example, using slurs to describe Lennie. (Contrast this to Curley's wife, who tells Lennie that he is "nuts). It's possible that I missed something, so by all means feel free to check my work.
Quite frankly, I think that you could criticize John Steinbeck for not portraying Curley as prejudiced towards people with disabilities. It seems unrealistic to me that Curley wouldn't be prejudiced.