We visited Las Grutas de Tolantongo late April, 2007. We stayed 2 nights in the hotel. Overall, it's a pleasant and even spectacular place, but it is no gastro mecca. We had one supper in the small, pleasant upper restaurant. The food was surprisingly good, although nothing unusual. Think of a U.S. National Park restarant. It's something like that.
The lower level cocina económica, at the base of the hill, was under construction at the time we were there, yet it was open. We had basic fare like huevos or quesadillas. Not bad, but nothing specila.
If you are spendng any time in Ixquimilpan enroute to the grutas, do some exploring and you may turn up some regional specialties. I had some good tamales and pretty good sopes at a small restaurant, El Retoñito,between Hotel Plaza Isabel and the mercado.
Inside the Ixmil mercado are some interesting looking barbacoa and carnitas de res (the last didn't appeal to me) but there's a lot more awaiting. I think the mercado has the greatest food adventure potential.
Enroute to Tolantongo we saw many signs advertising pulque at small ranchos. But we never saw any open.
You may want to stck up on snacks if you are staying over at Tolantongo. The restaurant and comedores have rather limited operating hours.