A Mexican pizza is a variation on the basic idea of a pizza recipe that substitutes several Mexican ingredients and seasonings, such as jalapeno peppers, salsa, and Mexican cheeses. There are a fairly large number of approaches used to make this kind of pizza, including some which only make changes to the toppings, while others change the crust and sauce as well. Mexican pizza is one of many different regionally-themed pizza variation ideas that have become fairly popular with the public, and different versions of this recipe are sold at a variety of popular restaurant chains.
In some Mexican pizza recipes, a regular pizza crust is used, but in other cases, a common substitution would be a flour tortilla. Sometimes the pizza sauce would be replaced with either taco sauce or some kind of salsa. Additional toppings that could give a Mexican pizza a distinctive flavor would be things like refried beans, meats that have been spiced with Mexican seasonings, hot peppers for an extra kick, and Mexican cheeses, either in combination with traditional mozzarella or as a replacement for it. Mozzarella has a fairly unique texture which can be hard to replicate, and this may be one reason why it isn't always replaced totally, even though it wouldn't necessarily fit with the pizza's overall theme.
There are also often some differences in the way a Mexican pizza is prepared in comparison to a more traditional Italian-style pizza. In many recipes, a lot of the preparation involves frying various ingredients on the stove with oil, sometimes including the tortilla-based crust. Usually, the recipes still rely on some degree of oven baking for the purposes of melting the cheese properly, but baking times may be shorter, and the overall preparation process may be a little easier in some cases than a more traditional pizza recipe.
Mexican pizza is one of many different regional and novelty pizza variations that have become popular, and it stands alongside dishes like Hawaiian pizza, which might be topped with pineapple and ham, and Chinese pizza, which might have stir-fry ingredients as a topping. The structure of a pizza can make it a useful platform for combining a bunch of ingredients into a single dish, and it often results in a food item that's easy to handle for eating on-the-go. This basic convenience, along with the general popularity of pizza dishes, may explain the large number of variations that exist around the world.