I looked this up as I hadn’t heard of it before and there does seem to be some danger in putting stickers on your helmet depending on the type of material it is made from (fiber glass casing seems to be ok). Nail varnish is not mentioned.
In terms of motorcycle helmets nail polish remover is meant to be bad but there are people who recommend nail polish to cover up scuffs and scratches to a helmet.
All in all… my helmet was nicked off my brother who no longer needed it and the nail polish was a solution to buying a nicer one. I’ll leave it, see if it breaks!
If it doesn’t, great. If it does, new helmet 