Stupid is not unconstitutional. California - LaLa Land - has a right to be Stupid. That's why people live there!
SCOTUS has no business approving or disapproving of California laws on the basis of whether Colleagues of the Wise Latina like it or not. And it is flat-out ridiculous to object to it on First Amendment grounds.
Which is why I commented more on vagueness (voiding laws for vagueness is a very old common law tradition). Stupid laws elsewhere are one thing. Vague laws are evil. Vague laws put people in jail on technicalities when people do not even know they are breaking them.
If California wanted a law that made sense, it could say no M rated games are to be sold to minors. That's another big government invasion, but it's a clear law, not without precedent, and easy enough to follow, just like laws against minors buying a 12 pack of Guinness or a pack of smokes.