Mustard helps stick the grilled pastrami to the burger and the pickles to the pastrami! So I vote yes.
But there are too many burgers to give such a generalization. Around here it's like Buford "Bubba" Blue - Mushroom burgers, black & blue burgers, bacon burgers, bbq burgers, turkey burgers, teriyaki burgers, avocado burgers, shrimp burgers, royal egg burgers, jalapeno burgers, pastrami burgers, stuffed bacon/butter burgers, deep fried burgers, Canadian blue buffalo burgers, taco burgers, porcupine burgers, Vidalia sweet burgers, PB&J burgers, pizza burgers, buffalo chicken burgers, nacho burgers, chili burgers.....not all are mustard compatible.
If at all possible I do like to mix mayo, ketchup and mustard for my fries, onion rings or crispy crowns.