Tokyo Disney resort is one of the most popular places in Japan. So it is difficult for us to find available restaurants in there.
“Guzman Y Gomez” the Mexican foods restaurant at Tokyo Disney Resort
In that case I recommend you to go out of Disney land or Disney sea temporarily and back to there after the meal.
Then my recommendation place is Ikspiari which is a kind of the shopping mall in Tokyo Disney resort near the Tokyo Disney land or sea.
Even there it is crowded during lunch & dinner time but there is some comfortable places as blow.
I think Guzman Y Gomez (GYG) is one of the most comfortable restaurants in Tokyo Disney resort.
Although the restaurant is very popular in Australia and US it is not so crowded since mexican foods are not familia with Japanese.
You can enjoy eating healthy and flesh Mexican foods in there.
Below link it the detail for the restaurant.