They are not generally "bad" per se, they're rather good, honestly. It's more like a meme because Weezer as a band do not take themselves very seriously, Weezer has made their name as a meme band because the music they used to make was considered "geeky music" that were written for virgin losers. This was back in 1990s, the "Pinkerton" album is the biggest example of it, which went to an entirely different way from the "Blue Album" which was considered "Goofy and funny". Can't ignore that Weezer also have a history of presenting themselves comedically, they've made weird YouTube videos to doing Fortnite Map. So yeah, they have a history of being susceptible to internet memes, hence the reason they became one. The problem started when newer fans came onto the scene and couldn't catch that the "hate" was just sake for fun and they started hating on them too and now there's a lot of actual hate present against Weezer.