Meteor Golem's ability is "When Meteor Golem enter the battlefield destroy target nonland permanent". Does Meteor Golems ability affect planewalkers, which count as players and not really creatures?
Yes, Meteor Golem can target and destroy a planeswalker on the battlefield. The player is a "planeswalker" only in lore terms. As far as game mechanics are concerned, "planeswalker" has a defined meaning, and a planeswalker object is not a player and vice versa.
A planeswalker in the mechanical sense is a type of object, a game entity that interacts with players and other objects:
102.1. A player is one of the people in the game. [..]
109.1. An object is an ability on the stack, a card, a copy of a card, a token, a spell, a permanent, or an emblem.
A planeswalker is not a creature, as you already noted, but its own permanent type alongside creatures and others:
110.4. There are five permanent types: artifact, creature, enchantment, land, and planeswalker.
Therefore, if Meteor Golem can destroy a non-land permanent, then that does include a planeswalker on the battlefield.