I put mustard on my jello.
I love mustard and have used it on Ribs and Butts.
I do not use it anymore and have never been able to taste the difference, however I never done a side by side comparison.
Yellow mustard is said to be helpful in Bark formation but I feel you get better rub penetration directly on the meat, again I have not noticed any difference in bark formation.
Some say using Yellow mustard compliments the meat with a vinegar taste, I use vinegar in my mop, so I would never be able to tell if the meat is flavored at all from the mustard.
My suggestion is, whatever works for you KEEP DOING IT!
For those of you that want to impart a mustard flavor.
Add Mustard powder to your rub and vinegar to your mop this may give it a little kick
Try tweaking a Carolina sauce with a little mustard powder as well.