Well, its not illegal. Like, the cops wont bust through your door (unless you live somewhere where that is illegal 0_0)
but no, its your game. Do whatever the #### you want.
As long as its not excessive and your game does not cost a thin dime (I'm looking at you EA) I say its okay
But don't expect people to be happy because everyone hates Ads (well unless you have nothing else to watch)