Mephisto is an extra-dimensional Demon who rules a fiery pocket dimension that he calls "Hell" or "Hades" although it is neither the Hell of the Christian religion nor the Hades ruled by Pluto, the Olympian god of the dead. ... Mephisto resembles the traditional visual conception of the devil.

Is Mephisto actually the devil?

It was later revealed that in that reality, Mephisto is not the devil, but the first mutant to exist where he is shaped by mankind's fear.

Is Mephisto more powerful than Thanos?

Being a deity, Mephisto has a plethora of powers, ranging from manipulating space and time to being so strong that no one has actually ever beaten him in the comics. It still looks like he holds his powers back and Thanos literally stands no chance against him.