I find the use of mainstreaming of drag performers very strange, and in most cases rather distasteful. For a zoo campaign, it is a weird choice, there's no clear connection, and I think drag is an adult form of entertainment.
However, it's not that they were saying a man is in fact a mother which is much more of a WTF. And happening in some instances as crazy as that is.
The question of why the entire gay community has come to be represented by a very small slice is an interesting one, and tbh I think a fair bit of the explanation for that is internal to the gay community. Rather than just outside people picking it up - there is some of that, and I've seen it at work for example with drag queen story-times, which often come out of direction from management that library programming, including for kids, needs to represent "diverse" people across all areas of diversity. How do you represent sexual diversity to kids, why not have a drag queen, they are like clowns a bit which kids love, right? And no need to mention actual sex which would be inappropriate.
Now its also just a fad, so follows the rules of fads where organisations jump onboard just because it is popular and they are trying to have wide appeal.