I spent quite a bit of time in Nogales, Mexico and ate as many tacos as I could get my hands on. Across the board they were far healthier than anything I can get from a fast food restaurant in the states. The tacos in (Sonora) Mexico generally consist of: Corn tortilla, meat, cabbage, onions, cilantro. Optional hot salsa.
I'm confident you can construct an unhealthy taco, but is there anything inherently wrong with tacos? Fundamentally it's just a bit of meat, veggies and cheese. The tortilla is grain and thus arguably not great, but tortillas are pretty small compared to the average sandwich bread.