Ketchup on canned tuna, yeah, I’ve done it. I’d rather have some mayo and sriracha on it, but it’s not that bad.
Ketchup on salad though, I can imagine it, I don’t remember if I’ve actually tried it, but unless we’re talking about sweet salads (like ones made out of condensed milk, all-purpose cream, canned fruit cocktail, gelatin cubes and the works), I don’t think it’s actually that hideous. Heck, I even think it’d go okay with something like potato salad!