I think I must be approaching this from the wrong angle. I have no problems at all when a fandom of mine goes temporarily mainstream. It means that there will be a lot more money poured into producing stuff in that fandom, which means a lot more choice of things for me to buy now AND a lot more things for me to pick up second-hand in a couple of years when the hype dies down.

As for the clothing thing - to be honest, I wouldn't say anything to anyone I didn't know who was wearing TF gear unless I was waiting for people to turn up at a meet, or maybe at a generic con.

I don't wear a lot of TF gear myself (and even then, only at meets and cons) for much the same reason, but then again I'm one of the minority who actually knows about fandoms and the people in them. I could totally see people who have zero knowledge of fandom just wearing something TF-related because it was 'in' or giant robots are cool, without ever wanting to strike up a conversation with a hardcore fan.

As a bonus regarding mainstreaming, more of the general public have at least a vague conception of "robots which turn into vehicles". I was talking to a bunch of 40-something folks the other week and mentioned that one of my hobbies was designing toys, "sort of like, you know, have you ever run into those toy robots which turn into cars and planes?" "Oh, Transformers!" "Uh, yeah!"

(Of course, part of that may have been that they were mainly engineers, but hey.)