One of the worst match ups for John I can think of. A weakness to fire vs. Loki who not only has substantial telepathy, but also can turn intangible and very often conjures flames and flaming swords? Negates a lot of his strengths.
Loki has probably used fire as a weapon more times than ice.
Loki's powers are more OP in a forum setting than Martian Manhunter's. The major edge John would have is his high end telepathy showings probably but Loki has invaded the minds of everyone from Odin (Through stealth) to bypassing Galactus' defenses with his spirit form (Gotta double check this one) to evolving a being into the world's most powerful mutant through sheer telepathy with a wink (More so than Professor X and Magneto).
I'm going to make a revamped Loki respect thread this summer. So much potential...