I've seen this with a lot of things, not necessarily things that I think that I'm better, just. Some people have the attitude that you can't critique media they like or you're critiquing them. Or they are enamored with the media and don't want to see the flaws or think they'll have to hate it. So if someone, rightly, says "this piece of media is flawed" (because there aren't any perfect pieces of media, even the ones that get close will have something that could be done better, whether that's something in the writing or something in the art style, or what have you), then the people who are devote to it need to believe that the critique only exists because of some bad intention on the part of the critic. Jealousy is an easy option when the critic in question creates similar media.
The people saying that you need to provide reasons for the criticism for it to be valid. eh. I mean, on the one hand, saying "the way this character is portrayed/drawn is racist because XYZ" or "Kiskia needs to do some serious practice, their lines are shaky and there's spots they missed cleaning up overdraw, and the forms they are using for bodies make them look lumpy in unnatural ways" those provide specifics about what it is you think means the work shouldn't have the following it does. But also, every time you think someone's art isn't as great as people give it credit for you don't have to do a complete formal analysis of why. You can just be like "I don't think the art's that great." And it could be that it's just not your preferred style, or it could be that there are actual artistic elements that could be improved.
I do think that if it's stated like "Why are they so popular? I draw better than them." or whatever, then that tends to invite more accusations of jealousy because you are making a specific comparison between your work and theirs and also bringing in the fact that they have popularity and you don't. Whereas, "Why are they so popular? Their art isn't great and there's a lot they need to improve on" doesn't have that comparison. But you'd probably still get accusations of jealousy even if the answer you're looking for is "they do this kind of marketing, they had this previous work to bring in pre-established fans, they had this pool of friends willing to spread the word for them" and it's a matter of they are popular because of the resources they had and random chance.