Mountain Dew does not have antifreeze in it so it's pretty safe to drink.
The antifreeze would be really toxic for humans to ingest so the Mountain Dew does not contain any antifreeze.
I have left a can of Mountain Dew out in the car on a freezing night and the next morning it had frozen and exploded in the car.
So if the Mountain Dew contained antifreeze it would not have frozen.
So no the Mountain Dew contains no antifreeze.
However Mountain Dew does have a flame retardant ingredient that is brominated vegetable oil which is a patented flame retardant for plastics that has been banned in foods throughout Europe and in Japan.
Many people drink Mountain Dew without any health issues so it's not that bad.
All soda is bad for you though in moderation.