Might I just say, SO WHAT?!
The difficulty of music has nothing to do with the quality of the music. Period. If you dismiss easy pieces as being trivial, childish, and pointless, you're basically just a snob that likes to show off.
I don't even really agree that much with people that say Mozart is more musically difficult than other composers, I think playing music by every composer requires the same amount of musicality. But it still doesn't matter because the degree of difficulty does not determine the quality of a piece.
It seems like most of the people who think Mozart is immature have not heard his very dark pieces, some examples being 2nd movement of k310, k475 (fantasy in c minor), and piano concerto 20 in d minor, k466.
And instead of bashing composers, why don't you try writing something yourself. I bet Mozart absolutely destroys anything you could write. I'm sick of all these snobs that just play the most technical pieces they can find and think they're great, but they've never written a piece of music themselves, which just shows how uncreative they really are. These people would be more accurately called "computers" rather than "musicians."
(I'm actually not yelling at you, pianoperfmajor, because I know you said that you appreciate Mozart, you just don't agree with people that say he's very musically difficult, and I sort of see where you're coming from. I'm more addressing people who just say Mozart is a simple, childish joke of a composer in every respect.)