In Overwatch can you play as Minecraft Creeper or Minecraft Herobrine? Asking for a friend. Skeleton Minecraft too. Slime Minecraft. SteveMC?

No, to all of the above. Custom skins and characters are not possible in Overwatch.

No offense, but have you ever played Overwatch before?

They have a roster of character's to select from, and you have to choose from that roster. And as far as skins go, they're not custom and you can only play with the one's that Blizzard has designed for the game.