Minnie Mouse can photobomb me.
Benedict Cumberbatch can photobomb me.
In both cases, I end up with a cool souvenir photo, with a celebrity in it. And I'm happy.
You? Making an ugly face? Acting a like a goof? I don't want you in my photos. All you're doing is ruining it.
The problem with photobombing strangers is:
1. You have no way of knowing if they'll think it's funny, or not.
2. You get a cheap thrill at someone else's expense and you give them nothing in return.
3. You're taking the coward's way out, by relying on anonymity to protect you, in the event they don't appreciate what you did.
So, save your photobombing for your friends and family, who presumably will appreciate it. And if they don't - they they can let you know.