Sticker Star stripped down the battle system to a very basic form; stickers were required for literally every action including basic attacks, as opposed to how previous games used their special moves. Bosses are a sort of forced-puzzle boss; you can either insta-win with the right thing sticker, or do an incredibly long slog against them. And one of the biggest complaints is that it pretty much simplifies the story down to nothing - for example, Bowser doesn't even get any lines, there are no Paper Mario exclusive species, and other such things. Basically it would be a mediocre game if it was standalone, but the fact that it's a relative letdown compared to the previous Paper Marios is what makes it a bad game in the eyes of many.