Malekith is one of the earliest examples of an MCU villain that audiences just didn't click with. He wasn't the first and he wasn't the biggest offender when it comes to bad villains, but he was the beginning of a noticeable trend of lacklustre bad guys. Thor: Love and Thunder suffered greatly as a result of this.
The great Christopher Eccleston was tasked with bringing the leader of the Dark Elves to life but that was darn-near impossible when the material he was working with was, for lack of a better term, dead. Malekith might have looked the part, but that was were his cool factor ended because he had the most basic of bad guy goals: To plunge the world into darkness and rule it. Yes, even a being as creatively out-there as a Dark Elf wanted to take over the world.
The first Thor movie had Loki as its central antagonist; a character flavored with intrigue and overflowing with nuance. It was always going to be difficult for its sequel to compete with that in the villain department, but it could have at least tried. The irony here is that Tom Hiddleston's 15 minutes of screentime as Loki here are easily the most engaging 15 minutes of the movie, making us all wish that the God of Mischief was once again the villain of this one.
The Dark Elves could have been an alluring faction of villains, but fans were so disappointed with this adaptation. Malekith had little substance and was simply supported by an army of foes with even less depth than him.