>>1524616it means the water jug isn't completely full and he wants it to be refilled. "topped off" means filled right to the top. if the jug were empty he would just say "more water" or "another jug/pitcher of water, please".
The fact that he's finishing his extremely specific series of demands with such a minor thing is meant to cement the impression that he's an extremely pedantic and demanding customer with no sense of self-awareness. if a normal person made so many specific demands, he would be embarrassed and think to himself "I'll get more water later, we still have enough for now. she has enough to be dealing with already". we're meant to conclude that this guy has no concern for others' feelings and doesn't even realise what he looks like. that's why we keep cutting back to the blonde girl who is obviously thinking "oh my god what is wrong with this guy? I'm so embarrassed, the waitress is gonna hate me" etc.
