Most Nacho Cheese is pasteurized so if you're pregnant and want to eat Nacho Cheese you can do so.
I ate Nachos and Nacho Cheese when pregnant and my babies turned out just fine.
If you're worried you can heat the Nacho Cheese up more to kill off some potential bacteria that may be in the cheese but usually the cheese has been pasteurized before it has been turned into Nacho Cheese.
Hard cheeses, Soft Cheeses, Mozzarella, Cheddar Cheese etc are all Pasteurized and are safe to eat during pregnancy.
Most nachos made with nacho cheese are made using cheese using monterey jack or cheddar cheeses so they are all safe to eat during pregnancy.