I don't know why she put mustard in it, truly I don't. Perhaps to go with the chopped celery and carrot bits..
and no, I met my husband when I was 34, we married when I was 37, and I had gotten over any need to eat mustarded jello to just get along. I didn't say "uggy poo" though, since I was a grownup: I just said no thank you, but I'll have another slice of 'whatever'. Her sons didn't eat her jello salads either, but her husband did, bless his heart. He was the better cook, pot roast and dumplings, for example.