Jessica Jones season 3 introduced a new character who is pretty much a mutant. The Marvel Netflix shows have come to a triumphant close, with Jessica caught in a dangerous game of cat-and-mouse with Foolkiller, and struggling to work out what to do with the newest vigilante in town. Fortunately, she has help in the form of Erik Gelden.
In the comics, Erik Gelden is a Z-list super-villain who calls himself Mind-Wave. He possessed powerful telepathic abilities, apparently either generated or enhanced by a strange helmet he wore, and his criminal career was ended when Daredevil unexpectedly teamed up with Uri Geller. The Marvel Netflix version is strikingly different; played by Benjamin Walker, Gelden is an empath who can sense the presence of evil. He's been using his powers for blackmail, but clearly has a deep desire to use them for good instead. By the end of Jessica Jones season 3, Jessica has put Erik in contact with her friend Detective Costa, and she hopes the two will form an alliance.
Jessica Jones season 3 strongly implies that this version of Erik Gelden is actually a mutant. In the comics, mutants are people born with the so-called "X-gene," which activates during puberty and triggers superhuman abilities ranging from the spectacular to the subtle. Erik Gelden's powers appear to be entirely natural; there's no mention of experimental procedures, radioactive emissions, or anything of the like. Furthermore, he also notes that his abilities developed back when he was a teenager. All this corresponds perfectly with the idea that Erik is the MCU's first visible mutant.
This isn't the first time the Marvel Netflix shows have made a gentle nod towards the X-Men franchise. Iron Fist season 2 featured two restaurants that were typically associated with the X-Men in the comics, and Daredevil season 3 featured a surprise cameo from Lord Dark Wind, the man who invented the Adamantium bonding process that Wolverine was subjected to. But this is the most important reference to date, in that it has profound implications for the MCU. It suggests that mutants could already exist in the background of the shared universe; they're just incredibly rare, and most of their powers are so subtle that the world hasn't really noticed them. Amusingly, for all Marvel Studios and Marvel Television have an infamously distant relationship, the TV shows may have helped set the scene for the MCU's version of the X-Men.
Meanwhile, there's an amusing irony to the approach taken by Jessica Jones season 3. Stan Lee originally came up with the idea of mutants because he was tired of having to come up with origin stories for his various heroes and villains, and he wanted an easier way of introducing characters. Now Marvel Television has neatly avoided giving an origin, and in so doing implied the existence of mutants.