There are so many restaurants to chose from on Isla Mujeres, the service here is nearly always good, the food can be rather variable. Some restaurants are really expensive others are quite economical, and regrettably some mislead the tourists. An example of that is La Bruja a local restaurant, where they tell clients that they have fresh fish fillets but they are in fact serving Basa, which is frozen and one of the most contaminated fish in the world.
In general however appetising it may appear, it is not really safe to eat food sold by street vendors. It may be part of Mexican way of life but being laid up for a week with a stomach infection is not the best way to spend your holiday. If you look, few have proper hand washing facilities and if they do it's the same water that they use to wash everything, it sits there all day and is prone to mosquitos laying their eggs there.
Villa Rolandi's hotel is a wonderful dining experience but check the prices first to make sure it's within the budget, they charge nearly $900 per night for a double room, which gives you a hint to the dining prices. The area where you will find most restaurants is the main street in the centre Hidalgo, Los Amigos, where they do whole fish cooked in salt, which is not at all salty, it's steamed, and Fredi's have very good food but both have seriously uncomfortable chairs; you can't beat plastic chairs for comfort. Dopi's is also very good especially for good quality steak cooked by the owner.
Many of those who have bought or renting Isla Mujeres luxury property, which is mainly in Punta Sur about four miles from the centre don't want to go into the centre to dine every evening, in which case there is Chilitos a basic local's restaurant which has excellent food and is full of Americans. Also Oscar's is a good bet especially if you have children because they do an excellent pizza and also deliver.
Try to keep a mosquito repellent handy when you dine out, they do a very good organic repellent in the large supermarket Chedraui. Where mosquitos mainly target is under the table, sometimes you feel them biting, other times you get home to find you have been bitten some thirty times. There is no malaria on Isla Mujeres but there is dengue, so it's worth taking precautions.