When Digital Live founder Steve Carroll put out the call for real estate agents to ride with him across Thailand in support of orphaned children, he was

What would an average to terrible X-men’s mutant ability be?

Rodney McNally has mentioned ‘The Worst X-man Ever’ and his ability to blow himself up (killing himself and those around him).

Others might be:
Cypher: ‘Understand any language’ which got pretty upgraded to the point he could read body language and know what people were going to do but was originally just ‘human universal translator’

Wraith: Invisible skin. You can see his insides, that’s his power.

In the same vein (pun intended) we have Glob Herman.

who, at least, seems to have super strength and immunity to telepathy to go with his ‘You can see my skeleton and internal organs’ power. (Wraith doesn’t)

Beak, again his ‘power’ is looking like this.

He’s also good at making friends, his power is looking like a monster chicken (and no he can’t fly).

Jazz

Again. Blue skin, that’s it.

There is a definite theme. Many mutants in the X-men comics don’t have awesome powers or any powers beyond ‘look weird so anti-mutant bigots know to target me’.

Edit: It’s been pointed out that, while not exactly combat worthy, Cypher’s power is awesome, making the ultimate hacker (among other things). I’m going to leave it there so the discussion makes sense but I might otherwise remove it.