I can answer those questions:
So Heimer can be considered an mediocre-average pick in that lots of other mages simply do what he does better. In high level of play, people know how to completely shut him down so it's very rare you'd see a Heimer picked in professional play or higher elos when you an just play Azir, Viktor, and Anivia. Ontop of that, he's also played less because his kit is less appealing than that of other mages of his role. Absolutely no bias, but I think that spawning turrets and empowering an abilitiy is just less appealing than flashier, more prominent mages.
So why is his winrate and his stats up? Heimer has a small dedicated playerbase, so naturally his stats will always be skewed towards the players that play him very well. Does that answer your questions?