To me, "a little mustard" on a sub means one line of mustard running the length of the sandwich.
→[More:]To the Subway "sandwich artist" who prepared my sandwich today, it apparently means "at least as much, if not more mustard than I would ordinarily put on a sandwich when a customer asks for mustard, so that it is impossible for DevilsAdvocate to eat the sandwich without getting mustard all over his hands and face." Sheesh. Frakkin' Subway.