That is certainly not true. The best tacos I ever tasted -- and second place isn't even close -- were sold by a woman who had a little carry out in the triangular space under a stairway off a plaza in downtown Guadalajara. The corn tostadas were handmade and kept in a large blue plastic drum. She put a teaspoon of some of ground meat paste of some kind on the tortilla, heaped a generous amount stingy and very, very thinly sliced lettuce over that, added some shredded queso fresco, and then poured on a very savory thin tomato based sauce. It was unbelievably good and each one cost about five cents. She served Pepsi Cola in sandwich bags which you would drink through a straw by holding the lip of the bag under you index and middle finger.
I've had tomatoes, cheeses and lettuce in tacos from all over Mexico, and in other places not, as you suggest. Another thing I can say is that the food in Mexico City always seemed to be very greasy as compared to other places.