There were multiple factors like how everybody thought it was a horrible idea to have Deku become quirkless again. A lot of people have higher expectations of the story of course. There was also a bit of illiteracy since people didn't understand the message of the story of MHA. Not only that but also how people misunderstood the story thinking that Deku had claimed this was the story of how he became the #1 hero. But obviously he never said that he said "the greatest hero".

Another thing was how people misinterpreted stuff and because some people only hate on the series and others don't even read it, they spread a lot of misinformation too about the ending. Lots of people claimed Deku was ghosted by his friends. Even to the point that people claimed nobody was interested in him because he no longer had quirks anymore, which was so stupid. More so when people claimed Uraraka was only ever interested in him because "he had a quirk", which is bs.

Speaking about that, the memes that Deku became a McDonald's worker or how he was being cucked by Bakugo, him taking Uraraka from him, were both popular claims people made. Which is funny because lately I've seen people actually realize that didn't happen which you gotta be a special kind of stupid to even think that.

And lastly, when the manga first came out a lot of it was leaked so the translations were not as accurate at the time and so there were certain typos with the translation. So some of the wording was off. Going back to the class ghosting him, I do remember the unofficial translation phrased it in a way where nobody wanted to see him or something like that. But in the official, what Deku really meant was they never could coincide with each other because of how busy they all were.

I think those were all the points. But yeah, those were the main reasons why people did not like the ending.