Generally its a matter of individuals bleating that their opinion is best. Thats what it really is, mature gaming attempts to apply a descriptor that ought not really be applied. It implies that one is probably better than another which is not the case.
As much as people complain about dnd, its no worse than gurps for high adventure. The point is having fun. Its always been more the people you game with, than the system that dictates success or failure in this regard.
The only wrong form of pnp is when people are not having fun at the table. Otherwise, you can be running FATAL, and if everyone is enjoying themselves it really does not matter in the least.
This quote from C.S. Lewis may help your understanding of my pov.
“Critics who treat 'adult' as a term of approval, instead of as a merely descriptive term, cannot be adult themselves. To be concerned about being grown up, to admire the grown up because it is grown up, to blush at the suspicion of being childish; these things are the marks of childhood and adolescence. And in childhood and adolescence they are, in moderation, healthy symptoms. Young things ought to want to grow. But to carry on into middle life or even into early manhood this concern about being adult is a mark of really arrested development. When I was ten, I read fairy tales in secret and would have been ashamed if I had been found doing so. Now that I am fifty I read them openly. When I became a man I put away childish things, including the fear of childishness and the desire to be very grown up.”
bad wrong fun is unhealthy, and specifically seeking adult or mature hobbies because they are mature is a sign of immaturity.