I think compared to mayo, the calories are negligible.
I like cranberry mustard on turkey sammiches. Brown or deli mustard at all other times. I never liked honey mustard unless it's made from Grey Poupon & honey. & even then, meh.
it might depend on whether they meant dry mustard powder, which has no calories, or the prepared mustard, which does have some calories, as they add oil and other ingredients to it, but in the amounts you would use the calories would have next to no impact - unless you are laying the stuff on with a trowel!!!!!
I love hot English mustard (hubby hates it), and honey seeded mustard, darn it I love em all!!!