Artie, the little kid mutant that hangs out with Leech is pretty useless.
Beak is a classic answer to this question.
Most Morlocks are just ugly and that's it.
When Bishop travelled back in time to the '90s he had a guy come along with him who had the mutant power to detect humans nearby.
Because he was so crazy Legion was once thought of as really useless.
Angel used to catch a lot of crap for only being able to fly.
Beast too before supergenius became his thing.
There are a bunch of mutants whose powers are only to effect other mutant powers, the lamest of those was a guy who was a "chronal anchor" for the time traveler Trevor Fitzroy, and this guy just kept Fitzroy in the timeline he wanted to be in when he altered history.
The mutants with uncontrollable powers suck hard, especially the ones with death touches like Rogue or Wither.
One mutant named "Courier" had the single ability to change genders, but not to shapeshift into anyone other than two versions of themself.
Did I mention Toad?