Thor knows a tiny bit of magic. He can speak the incantations required to initiate the god-blast and to summon berserker rage.
Although its less relevant to Marvel's Thor, the reason magic was not widely used among the Norse gods was because the vikings considered it unmanly.
Worship drifted away from Odin and towards Thor for several reasons, one of them being Odin's use of magic ('Seid' or 'Rune Magic').
Could Thor learn magic as well as Loki or Dr. Strange? Probably not.
Part of successfully learning a skill is an interest in the subject and to meet a goal that requires the dedication. Since Thor generally surpasses the greatest Asgardian-affiliated sorcerers, he wouldn't have the necessary drive to become as accomplished as Loki.
Loki's rise power is due (in part) to his absorbing power from his former mentor. He took a short cut. It's possible he may not have surpassed Amora or approached Thor-level power without doing so... who knows.
Karnilla has some affiliation with the Norns and learned their secrets. Odin underwent considerable sacrifice, and his sorcery is backed up by the Odin-Power (note that King Thor, outside of Oeming's version, never really uses magic as Odin does). The Enchanters, possibly the greatest sorcerers in the Asgardian dimension, employ talismans to augment their sorcery.
IOW, it would be a waste of Thor's time...
So who would be considered to be their greatest magic user, Karnilla now?