"Nach e neach-frithealaidh sgoinneil a th' annad?"
Translation:Aren't you a brilliant waiter?
Could be Canadian English.
I asked a few other Canadians and they had the same reaction.
A negative question implies asking for a positive answer. Asking someone to given themselves a compliment rather than you giving the compliment has the implication that they have a higher opinion of themselves than you have of them. ie. "I think you did a good job" as opposed to "You think you did a good job, don't you?"
There's also the fact that "Brilliant" is not used in Canadian English the way it's used in British English. That's going to make this use sound more shady.