Yes, Testors clear coats could yellow over the years, for the older stuff. They seemed to have fixed that several years ago. Sometimes frosting is paint blush. Blush is caused by rapid evaporation of thinner on very humid days. Lacquers are particularly susceptible to it because of their quick dry. I suppose acrylic lacquers could blush too.
Blushing happens because the rapid evaporation cools the paint surface down below dew point, and water (dew) condenses on the surface giving a rough surface. For clear coats the rough surface makes them look whitish and matt. Never paint with a laquer solvent paint when the dewpoint is close to the actual temperature.