"do you think it should be legalized or illegalized"
Criminalize Fandoms. You have got to be kidding me. There are two concepts, Freedom of Association and Freedom of Assembly, that make such a criminalization not only unlawful, but a violation of basic human rights to join or leave a group of your own free will, with a clause in some versions of declarations or laws pertaining to these concepts about military or criminal activity being the focus of said groups. You're mileage may vary from country to country.
And yes, even though the above two are meant to protect political groups, they have been extended in the past to cover things such as Sports Clubs, which are pretty much the early forms of fandoms. So you'd need a pretty good reason to say "Expressing ones interest in a topic with others who also enjoy discussing said topic is illegal".
Outside of that nonsense, I think Fandom's are important. They are an outlet for which people can express ideas, thoughts, opinions, their imaginations, and especially express their feelings with one another in a usually healthy way. They give a unifying common purpose that isn't something so serious to be devouted to. In a way fandoms are like any other massive group of people who like the same thing, it can be a varied landscape of different people brought together or an echo chamber, but even then still provides an outlet for these various feelings and pent up energies to go. Like, do you really want the bad parts of fandoms bringing what they normally bring to the fandom into non-fandom stuff like religion or politics? You really want that guy who screams at people over episodes of a cartoon or lets play putting their energies to affecting your daily life?
I happen to think fandoms are giving people a way to get out that negative emotional expression, while at the same time providing a medium for people to find positive encouragement from other human beings in ways they may not be able to in real life. And while this can produce its own brand of cringe, I'd rather people find emotional support through common interest, then live a life where they feel absolutely alone and a constant outsider. I think we all are mature enough to know where that kind of path can lead.
So in my opinion, Fandoms are a positive force, both for expelling the bad and enabling the good. Like life, it is a kaleidoscope of different people, good, bad, saints, scumbags, hard workers and lazy whiners. They are not and probably will never be seen as serious or important, which is a good thing to keep them as the non-harmful outlet they are. But saying we need to get rid of them is the shortest of short sighted things.
Anti-fandoms though, that's a whole 'nother mess to go through. Mostly because the positive enabling part of the fandom is foregone in order to create a bigger space for the negative expression. I'll be the first to shit on something I don't like. But joining a group dedicated to hating a group of people is a tenuousness step towards the wrong direction.