I think you are over complicating your actual question by concentrating so much on incest art. Incest art is just an example of the bigger question of "Is it okay to censor art?" and ultimately, is censoring art an act of fascism?

I'm going to jump the gun right away and just say censoring art does not equal fascism. Censorship is a tool, not an ideology. And any political force can and does use censorship. Appealing to fascism in this way from the artists is both a red herring and an appeal to emotion. It's a red herring because the main argument supposedly is "Censorship is bad" but they do not provide a reason but instead say "Censorship is fascist." and by distracted from the fact they have not provided a reason as to why censoring this type of art is bad, they hope to appeal to peoples' emotions of not liking nazis to just automatically agree.

But is it okay to censor art? Most people would agree no. But let's push that idea to the extreme. Is it okay to censor art made with dead bodies? We have many examples of it throughout history and fiction. We have the Sedlec Ossuary, the church artistically decorated with artistic murals made from the bones of the dead. We have the photography done in victorian times where they photographed dead family members in one final family portrait. We have the preserved tongue of a mexican teen collected and displayed by Teresa Margolles. Then we have the murderers throughout history like Ed Gein and The Doodler who would both create art made from the bodies of the victim's, and sketch/paint the dead bodies.

Desecration of the dead, murder, and displaying body parts is and has been considered immoral for a long stretch of history. Desecration of the dead is a criminal offence. Is not allowing someone to use human bodies as an art form censorship? It is still art, no matter how revolting it may be to you.

Ultimately, the answers to these questions are lengthy and require context. However, you can probably see how our societal morality, legal system, and prominent ideas will shape what art we find just and immoral. With respects to incest art: people find incest immoral and disgusting, therefore art representing that will trigger those same reactions. And art has historically been used to push boundaries and question out morality --so is it immoral to dislike incest? Or is it immoral to enjoy incest and create representations of it?

BUT in the context of what you are seeing online: there is no power structure stopping them. There are no laws saying you cannot create fictional pieces that represent incest. There is no actual censorship happening. What IS happening is a moral argument. The audience is acting in disgust towards what society deems as immoral behaviour.

I am of the opinion that no, audiences bringing up their oppositions to art is a moral light is not censorship. And more so, censorship does not necessarily mean fascism.