I can understand comparing Bayonetta to a disease, frankly just because of how much hate she can generate.
Let's put it bluntly; she is the most HATED character in the game for a multitude of reasons, with Cloud and Sonic not to far behind. As for the argument that she "isn't that good", well that's just frankly wrong, despite her in my opinion being somewhat overrated and she has more 50/50s, though that's not saying to much in a game with such a large cast. The fact that you have to fight her in a specific way compared to other characters, and that fact that she is ironically hard to punish, despite that being a key weakness the developers put in, given that players have ways around said weakness, makes her genuinely infuriating to fight against, since you can't do things that work on literally every other character in the game, for risk of over extending, it's only guaranteed things that work pretty much, and very careful reads. Many people I'm pretty sure want her nowhere close to the next game, and in many regards, she really does feel like a Marvel character in Smash.
Even Cloud, as basic and ridiculous as he is, is still pretty much fought like other swordsman, and he has more key weaknesses that, while they can be worked around, are still heavily exploitable.
It doesn't help that Bayo's also pretty much the only character in the game where SDI is practically REQUIRED if you want to have a decent chance, and even then if the player reads that, Bayo can continue to rack up damage if she get's a good read. Bayo's weaknesses, while exploitable, are still easily able to be worked around, and even then she is such a hard character to fight against that more often than not, it doesn't feel worth it.
Plus we should also consider the possible psychology to it.
1)She Won the Smash Ballot: Like it or not, she won the Smash Ballot, but even then, her inclusion was just as divided as Corrin's at her release. Corrin at least made more sense, given she/he was a Nintendo character and a character used to promote a new entree in a long running series that had started to gain way more attention(You could argue, why not Inkling then, remember, Splatoon at that point in time was brand new and hadn't really been established by the time Corrin began development). Plus we can assume that the ballot likely only had one winner, that said, for that winner to be a character that frankly, not many people thought it could be/ who it should've been, may have something to do with it, not the main cause, but something to consider that generally, there were many people that didn't want/care for these characters.
2)People Hate Feeling Helpless: This is something quite a few characters in this game are good at, but Bayo not only does that, but she makes you feel like crap, simply because a character with a built in weakness, is ironically hard to punish and has a few ways around her key weakness. As such she does such a good job at not only getting her way a lot, but she's also the best character at making sure you can't get your way.
3)People Hate when Other's "Get it Easy": This ones a bit odd, but it has some merit. Compare Bayo to Sheik, both are clearly Top Tiers, but Sheik is one of the most technical characters in the game and requires MUCH MORE effort to play effectively than many of the other top tiers. Now compare this to Bayo, who is able to perform these deadly combos with relative ease, and only has to worry about DI and SDI, as apposed to messing up timing,spacing,pivoting, inputs, etc. Cloud is definitely the worst offender, given that he's one of the best characters in the game, and he require's little effort to pick up and do well with. One of the reasons why people don't mind Fox being the best in Melee, may be due to the fact that he is an extremely technical character, and one miss input can severely screw you over. With Bayo, while she is harder to play than some of the other top tiers, comparatively speaking, there are many other characters in the game that require more effort than her.
Comparing Bayo to a disease, well, I can understand the justification, but I also don't think that it's healthy for this community as it can become something far worse. All we can do is offer our own point of view, and hope that one day, we can move past this. As it stands though, if she does make it into the next iteration of Smash(which if I'm being honest, I really hope she doesn't), or we somehow get a new patch, she will most likely be drastically toned down due to the hate that surrounds her.
is not bullcrap, she has lots of weaknesses that can be exploited. We talked about said weaknesses earlier in the thread.
to do it. 
