I think you might be overestimating how much insight the waitress has into the business. It may be the case that this was a close, family-held restaurant and they have a daily pow-wow on cost of goods and marketing strategy, but I wouldn't bet that way. My guess? Groupon customers are poor tippers, either from adverse selection or because they either a) attempt to tip with leftover funny-money or b) tip based on the…
Here in Czech republic we have local variants of Groupon and in some restaurants people reported the despise from waiters too. Surprisingly, this did not happen when we went to one that was otherwise really expensive - the people there were very nice and friendly. We won't return there regularly because it's out of our price range but for special occasions I would like to go back.
Regarding the quality - from my experience it's about 50/50, some restaurants are bad, some are ok so maybe contactdick was unlucky in encountering the wors first.