If one would believe Joe Nocera, former op-ed columnist for The New York Times: No.
In an article titled The Harper Lee ‘Go Set a Watchman’ Fraud (July 24, 2015 - the book was published on July 14, 2015), Nocera claims that this is a money grab from Lee's current protector, Tonja Carter.
Harper Lee: The Sadness of a Sequel (February 3, 2015) in The Atlantic notes:
Harper Lee, née and known to those close to her as Nelle, spent the
majority of her life not wanting Go Set a Watchman to be published.
Or, at least, she has spent the majority of her life telling the media
that she didn't want Go Set a Watchman to be published.
This isn't the first instance of controversy after Carter took the reigns. A biography of Lee by Marja Mills also ran into trouble. How Unauthorized Is the New Book About Harper Lee? from Gawker (July 18, 2014):
Things formally went sour in 2011. It was a transition year for the
aging sisters, for one thing. Alice Lee, who had practiced law past
her 100th birthday, finally retired in December and went into a
nursing home herself after a bout of pneumonia. It seems clear that
over the course of the year she'd been handing off the reins of her
law firm to a younger partner, Tonja Carter.
Earlier that year, in April, Mills had finally sold her book. When the
sale was announced, Penguin Press (an imprint at Penguin Random House)
described the book as having been written with the participation of
both Lee sisters. But shortly thereafter, that same month, a statement
bearing Harper Lee's name reached the press:
Contrary to recent news reports, I have not willingly participated in
any book written or to be written by Marja Mills. Neither have I
authorized such a book. Any claims otherwise are false.
Mills seems to have been caught off guard by this. She already had in
hand a statement from Alice Lee dated more than a month earlier, in
which Alice confirmed both her participation in the project and her
Alice Lee passed away about 8 months before Go Set a Watchman was published.
All said and done, it looks very fishy.