Do players to pitch a skin/champion idea that gets made get that skin/champion for free?

I dont think they have ever made a champion based on the community.

For Skins its one of the reasons as to why its so rare for them to have a community made skin actually be made. I know the Surprise Party Fiddlestick got it for free, as well as Battle Bunny Riven. im sure there is more. But ya if you did make a skin that actually becomes a skin you will get it for free (as well as the champion if you dont own it i assume)

But again thats why its such a rare thing to happen. So dont hold your breath.