Carbonated soda has a pH of about 2.5-3. I doubt very much that the mouse would be dissolved. It is however about as acidic as apple cider vinegar. (Ask your dentist!)
This is likely a hoax and lawyer bait, but I don’t much care for the BS response from Pepsico either.
But I happen to have some Mountain Dew and work in a mouse-infested building, so let’s play Mr. Science!
PLEASE take time lapse photos, if you do~!!!!