I think that ketchup can go on anything. Will I eat it afterwards? Nope.
The absolute ONLY food I ever put ketchup in by my own volition and with the intention of subsequently eating it is the cholent.
My sister, on the other hand, is an advocate of the “ketchup-on-ketchup” school of thought. Her favorite sandwich, until recently, was a burger with ketchup and pickles with extra ketchup and no burger. Sometimes she would add sliced tomatoes for that bit of zing. My brother, following in her footsteps, ensures that it really is worth it to buy those gigantic ketchup bottles at Costco.
What food shouldn’t have ketchup? I don’t personally care, but if my dad ate ice cream with mustard, it’s only a short step downward to ketchup. I should advise him to try it.