Loki, Asgard's Mxyzptlk
This issue is bad.
The writing is bad, and the art isn't good enough to save it.
It is the first appearance of Loki, so it gets points for that - but that's it.
Loki, who's trapped in a tree on Asgard, can only be free if someone sheds a tear over his plight. Using his powers to make the tree lose a leaf right as Heimdal is passing, so that it falls in his eye and sheds a tear (which was caused by Loki's plight at being the one to make the leaf fall - don't think about it too hard, it'll just make your head hurt) thus freeing him. So, yes, the story begins with some crazy Deus ex Machina, and it's only the first of several.
On top of that, Loki comes to Earth, where he battles Thor with an unending array of whatever-he-can-dream-up powers, making him kind of like the Mr. Mxyzptlk to Thor's Superman. Only if Thor outwits him can he be sent back to Asgard.
Then there's all sorts of 'conveniently-we-tell-you-now' moments, like, Thor cannot be separated from his hammer for more than 60 seconds, or he reverts to Don Blake; or Loki's powers don't work underwater.
But the inanity doesn't end there - here's just a few samples of the dumb writing:
"Those people -- they've all been transformed into negatives!!"
"If I rotate my hammer fast enough, it will emit anti-matter particles!"
"I've maneuvered him into a position where the sun's rays reflect off his hammer into his eyes each time the hammer passes in front of him! The blinking reflection plus my power of suggestion will soon have him hypnotized!"
And when Thor refuses to give Loki Mjolnir willingly, Loki tricks him by pretending to be him:
"Behold Thor, the Mighty -- the Thunder God! The hammer is his! Give it to him!"
There's even more annoyance in the form of narration blocks that randomly break 4th wall, and of course, everything about how Thor actually wins. Add to that none of the cool distinctive Kirby art style, and you have an issue that should chained to a rock by entrails as ink is dripped over the pages to obliterate them from existence.