It’s their job to be friendly with you. However, if the waitress is going out of her way to talk to you, compared to what she is doing at other tables, that might be a sign of something more especially given her time constraints. I would not say that most waitresses “flirt” with their clientele for better tips. Friendliness, yes, but not so much flirting. A lot of men take very basic friendliness as flirting. If she is clearly flirting, I’d say that could also be a sign of something more. Now, she might go along with your flirting just to not offend you, but if she comes on to you, I think it has a good chance of being something more. Waitresses take the risk of reducing their tip by fake flirting in an obvious manner and it’s a costly pain to be inauthentic all the time, so being friendly is what they usually resort to if their concern is solely for their tip.