People who prefer ketchup over mustard are annoying because as soon as you say you like mustard, they go on and on about how much they hate it. Like, okay. You have the same flavor palette you had when you were 5 but that doesn’t mean you should insult what I put in my coffee.
I just tried this (I had to know, I’m sorry!). I won’t be making a habit of it, but it wasn’t _that_ bad — the mustard mostly just went to the bottom and only had a minimal effect on the flavor until the very end.