it is not about it being toxic or rashes, it is about breath. especially during hot times like summer.
skin can inflate due to inability to sweat, or cause skin to prune and rupture under the paint if left for long times.
it can also trap heat more likely to have heat stroke or heart attack as you get older.
proper skin paint can be left on for much longer times. and can sometimes be water resistant, for times you need to hide in the sewers or jump in water to eat your next victim. regular paint must also be washed off and reapplied more often, costing more in the long run if you want to keep your nightmares running into your more advanced ages. and as we all know, real monsters are immortal. think of the savings.