While it may not necessarily be "superhero fatigue", it could well be correct to say it's "Marvel fatigue". It's not like Disney haven't been pumping out content for the franchise in enormous quantities, to the point that even those who don't really follow the franchise are aware that it's absolutely massive and there's a lot of content you have to consume if you want to actually know what's going on. This is always a problem with big franchises: either you have to consume all of it (which means you get tired of it quicker), or you have to skip some and then be confused later when suddenly there's a character or enemy or event or whatever that you're supposed to know all about, but you don't because you didn't watch that other series/film.
Superheroes aren't my favourite genre, but I like to dabble every now and then, and there are some superhero TV series and films that I have genuinely loved. But frankly at this point I wouldn't even know where to start with Marvel because there's literally too much of it. Keeping up with a franchise shouldn't be a full time job. But Disney is essentially assuming that everybody has time to watch everything it pumps out for its franchises, but somehow simultaneously never go "you know what, I've watched a lot of this lately, I'm in the mood for something else." The more stuff becomes required viewing, the more of the audience you lose due to people just not having enough time.
People could well be experiencing some Marvel fatigue without feeling superhero fatigue, just like I feel a little Star Wars fatigue while still being interested in other sci-fi. Disney want a monopoly on entertainment, but they also don't want to risk spending money on a wide variety of franchises in case some of them make a loss, because the short term losses on a few failed experiments are more important than the long term gains of creating something new that endures. So all they do is recycle the same stuff over and over, oblivious to the fact that audiences won't just keep buying the same stuff over and over. Marvel, Star Wars, remakes of animated films from 30-80 years ago... Disney won't take risks anymore, so they've over-saturated their own market with repetitive products that consumers are losing interest in.