I had never had a problem with mosquitos at WDW -- which is odd for what essentially swampland. I figure they treat pretty heavily.
However, we stayed at the BCV earlier this month and I did get bitten a few times when walking in the evening along the back path from the Villas to Epcot, and in the courtyard outside of the Marketplace. So, it may depend on where you are staying and what you're doing. I think, in general, the parks and most of the resort areas are fine.
(Please also note that mosquitos find me DELICIOUS. I am a mosquito magnet. We can be out somewhere and I will be the only one who gets bitten, and I'll have bites all over me.)