Hi JG,
There is probably a higher concentration of mosquitoes (and all manner of critters) at lower elevations where it’s generally warmer and, especially, more humid. However, even at altitudes of 5500-7500 feet, which covers most of the central highlights of Colonial Mexico, you’ll encounter plenty of mosquitoes. They are more prevalent in the spring and summer, when temperatures rise and they are particularly active during the rain season, see: https://www.mexperience.com/mexicos-rainy-season/