I'm sure some people in various fetish communities are mentally ill, I just don't think its very beneficial to focus on that aspect. From the fetish communities I visit, most of them are bombarded with such accusations and they find it frustrating.
I will say though, the people who are actually being open about their various attractions and being activists are probably more prone to being mentally unbalanced compared to the rest of their group. To admit to various fetishes in public will often resort with backlash and lost friends and the minds who are willing to go through that fire might be aided with a less than sound mind. Plus, many people in the fetish communities lack proper psychological care. Not that their fetish itself makes them ill, but they're born in a world that generally does not understand them and as a consequence they have little resources to provide them guidance so over time some might develop mental health problems.