Never said it wasn't one. Just never heard of it. Wasn't aware it was considered obscene either. Will look into.
It's got some depictions of violence that I think parent groups object to, so that kind of obscenity. There's technically nudity in some of the dead bodies I guess?
Maus was subject to one of the many ongoing book bannings in the United States and was removed from multiple libraries due to containing "adult content."