There is no Mexican themed restaurant in a WDW resort. There are, however, yummy Mexican options in other areas of WDW. The place that comes first to mind is the Mexico pavilion at Epcot. This part of the World Showcase includes both a table service restaurant,
San Angel Inn, and a counter service restaurant, Cantina de San Angel. These restaurants offer classic Mexican fare like Camarones al Ajillo and Mole Poblano and Tex-Mex dishes such as burritos, nachos, and quesidillas.
Another restaurant to explore is the brand new
Paradiso 37 located at Downtown Disney. This location features foods from a variety of Latin countries, including Mexico. Dishes here include burritos, enchiladas, and tacos. There is no admission fee or ticket required to visit the Downtown Disney area, so you can eat there without using a park pass.