As a mexican, I can tell you that that's how mexican hot dogs are always served. Chopped tomatoes and onions are always in a mexican hot dog among many more toppings depending on the region you eat them.
I had installed Internet for this one Mexican lady and she gave my partner and I some of the best dogs I've ever had. It didn't have pico, exactly, but the tomato and onion at least, along with carrots and radishes, mayo mustard, bacon, and more... this thing was like a salad on a bun and I wasn't too sure about it at first... But I was super hungry at that point and took a small bite. Fan-fucking-tastic. I wish I ate it a little slower so I could remember everything that was on it because damn it was good!
I went to a baseball game last year and we had club level tickets with a buffet. The hotdogs were next to the Pico, so i figured why not? Great decision. Been making it at home since!