Easily Mutants. Skrull can sim appearances but not powers. Professor X and Cassandra Nova alone could probably take on a horse of Skulls with ease. Skrulls have superhuman strength, speed, and stamina, but that won't help much against a telekinetic and a psychic.

Normal Skrulls? Easily Mutants and it's not even close, Kamala alone probably could took out most of them and she's probably one of the weakest Mutants, When you add Insane OP characters like Jean Grey and Storm (Who has powers that can rival Freaking Wanda at her strongest) and better fighters like Logan and Cyclops on the teams, unless every single Skrull is copying someone like Odinson, the Skrulls stand Zero Chance.

Super Skrulls however, It's a much different ball game. Let someone like G'Iah gets the DNA of Jean Grey or Professor X and see what damage they can do or Logan Insane Broken Healing factor, let's just say that Nick Fury should count his lucky stars that the Mutant wasn't there at the final Battle at Endgame or else G'Iah and Gravik might be unkillable.

Mutants. Phoenix would destroy most of the Skrulls.

only one ability for Skrulls? if shapeshifting, Secret Invasion show seemed to imply that Skrulls could be upgraded.

as for which species would win in a fight; it could potentially differ from time to time.

though the side that does any of the following might have a better chance at winning::

• take advantage of the environment.

• exploit rival's weaknesses.

• work with any allies and/or with each other.

The Issues is that for the X-men Team they really don't have any weakness to exploited, Phoenix simply can't be killed (The Fox Universe doesn't count, they don't know they are doing with Jean for the most part and they Nerfed Jean badly in their universe lol) Plus Rogue can just take whatever powers the Skrulls picked up.

The Only way they Skrulls are winning is if they all Shapeshift into someone like Odinson.

Mutants