Would many interpret this mechanic's "humour" as "arrogant" and acting "wiser" than you?

Sounds to me like he was simply trying to lighten up your mood, and you read his tone of voice wrong. I worked with customers all day long when i was working in the shop. Saying "hello, what can i do for you" not only gets REALLY old fast, but sounds too formal, People respond better when you treat them like real human beings, not just a source of income.