There are few superhero teams with a rogues gallery as vast as the X-Men's. Their 50+ year history has led to some of the most iconic Marvel Comics villains (Magneto, the Sentinels, Mystique) of all time, but a history that deep is also hiding some baffilingly dumb villains as well.
What else can you say about a mutant who can turn themselves into any kind of ice cream imaginable other than "...huh?" Whether silly, boring, or flat out ineffective, these villans give mutantkid a bad name.
We all know the Sentinels as the anti-mutant robot task force, but it turns out that the computer that creates them is really...a slightly larger Sentinel. MM was originally built by Bolivar Trask to keep humanity safe, but like every other all-powerful computer in fiction, it decided that taking over humanity was the best option.
Onslaught's lameness isn't entirely his fault. He just so happened to be involved in one of the most desperate attention grabs in Marvel Comics' history. The product of Magneto and Professor X's subconsciousnesses over here killed off the X-Men, Avengers, and Fantastic Four, only to be retconned shortly after. In design and powerset, Onslaught was cool enough to be the final boss in the first Marvel vs. Capcom fighting game. It's too bad his original story didn't do him more justice.
As far as Marvel's list of contract killers stretches, Arcade is probably the most bizarre of the bunch. A techno-genius who traps heroes in elaborate carnival-themed "Murderworlds," Arcade is rarely ever able to close the deal when it comes to killinng anyone. X-Force even wound up using some of his old haunts as training grounds before he destroyed them remotely. Arcade is a prime example of all flash and no payoff.
Just...look at this guy. A mutated geneticist from the alternate Age of Apocalypse timeline, Sugar Ma's claim to fame is working under that timeline's Mister Sinister and jailing Illyana Rasputin aka Magick. He managed to escape the timeline by shrinking (he can change his size and mass at will) and hiding in Colossus' boot. Taking his appearance and vast powerset (superhuman strength, genius intellect, those sharp claws, teeth, and tongue) into consideration, Sugar Man should be more threatening than he turns out to be time and time again.
Black Tom Cassidy is the cousin of X-Man Banshee, can project energy through wooden objects and is more bitter than you can possibly imagine about it. So bitter, in fact, that he kidnapped and raised his brother's daughter Siryn to become a member of his gang. But Cassidy's main claim to fame is being Juggernaut's sidekick for a number of years, eventually gaining the power to control plant life as well as rely entirely on wood to use his powers.
How can any clone of Cable wind up being a dumb character? Look at that outfit, for starters. It might be great armor, but that headpiece is making my eyes hurt. Stryfe was kidnapped by Apocalypse as a youngin to serve as a new host body, but daddy didn't want to use him after it turned out he was a clone; so he turned on both his adoptive father and his "real" parents Cyclops and Jean Grey. Even with all of Cable's powers (and none of the techno-virus stopping drawbacks) and the entire Mutant Liberation Front at his disposal, Stryfe managed to be little more than a thorn in anyone's side at any given time.
A mutant who can turn himself into any flavor of ice cream you can imagine. Great at parties, less great when it comes to breaking into houses. He only appeared once back in 1983 when he decided to destroy the X-Men because their powers made him look stupid. He was frozen by Obnoxio The Clown for his troubles.
And here we have an eyeball with a human body and a ray gun. That's it. He attempted to rob a bank in the Astonishing Spider-Man & Wolverine series, but was apprehended even before Spidey and Logan arrived on the scene. What's a giant eyeball to do?
You would think anyone with the powers of Goro from Mortal Kombat would be a happy camper, but in the complex world of X-Men, you're about as effective as a dry sponge. So is Forearm, a mutant with four arms. Yay(?)
Stonewall is a mutant with the ability to control his mass to withstand any attack - as long as he's level with the ground. This didn't come in too handy when Storm threw his ass into a river and he nearly sank like a stone.
An energy spear and a bionic peg leg are all it takes to get some in a Moby Dick kind of mood. Rory Campbell aka Ahab was a jailer in the Days of Future Past timeline who was responsible for turning Cyclops and Jean Grey's future daughter Rachel Summers into a mutant-hunting "Hound" before he was mangled on a heap of computers. Call him washed?