Can proliferate be applied to loyalty counters?
Yes you can proliferate loyalty counters on planeswalkers because they are permanents. 

Planeswalkers are not players however, the only time they sort-of act like players is "If noncombat damage would be dealt to a player by a source controlled by an opponent, that opponent may have that source deal that damage to a planeswalker the first player controls instead." And really that is just a rule so that cards like Lightning Bolt will be effective against them without needing errata. 


  110.4. There are five permanent types: artifact, creature, enchantment, land, and planeswalker.