The stupider the customer, the more flirting they'll receive. You can usually tell who the stupidest customers are. They're the ones who don't order much food, but their wallets still end up empty.
So, yes, they do flirt. Nothing sexual, usually, but a lot of shoulder touches, familiar-sounding nicknames (Sweetie, Sweetheart, etc...), a few well placed compliments, some exaggerated fawning, and they'll be really good at pretending that they care about what a customer says.