In my opinion, Mensa is a mishegas wherein people who do well on a type of test get to meet and say we are better than everyone else. It’s primary purpose is the gayva of its members.
I don’t equate high IQ with = very smart, but I don’t think there is ever a downside to being smart. The smarter you are in a given area the better for you in that area. Some people are smart in some areas and not in others. If those people are smart in, for example, math, and not smart in interpersonal relationships, it might give the impression that being too smart is a negative. But in reality it just means that they are not smart enough in areas that matter most.