There are fans on both sides of the Donna dispute filling up the That '70s Show subreddit. Some of these fans, as evidenced by the thread entitled "What happened to Donna?," think that Donna's character has grown in ways they find upsetting.
"I'm on Season 3 and oh my god she is just not the same, cool, chilled out Donna from Season 1. She's p***ed at Eric every time he has a thought and is just whining about everything all the time ... Jeez what happened to her? Tell me she gets better," wrote u/misanthropeint, the original poster on the thread.
u/ThatsMyBounce echos a similar sentiment, writing, "Donna had some pretty cool moments in the first couple of seasons of the show, but then her character went to s**t. She turned into a hypocrite who thought she was always right, had to have the last word, would storm off in a strop and quietly agreed with the people who thought she was a lot better than Eric."
u/misanthropeint cited a couple of particular incidents that highlight this perceived shift in Donna's behavior: In season 2 when Donna tells Eric that he couldn't possibly get with a hot blonde classmate (Shelly), and in season 3 when the couple is writing an essay together and Donna tells Eric that he's wrong and she's right.
"Between Seasons 1 and 2 where the writers took the personality of someone who was deeply flawed but entertaining and redeemable," u/misanthropeint wrote of the exact moment writers let Donna's character go, "and made her into someone who is deeply flawed but thinks she's God's gift to humanity by Season 2 and onward so far."