It’s not so much overkill as a little odd.
Electrical stun weapons require ongoing contact to function properly. So, your character would need to punch someone, and then… just, keep, standing there holding their knuckles against the intended victim, to get the full effect. It’s not a deal breaker, but mounting them on the knuckles is less than ideal.
The second thing is that you need direct contact with the victim. Most stun guns can be subverted by wearing heavy clothing. If the prongs don’t fully penetrate, then you’re not shocking the victim, and the weapon isn’t doing anything. Because of how punches work, connecting with sharpened studs to produce general tissue damage would probably be more effective than electrifying them.
Also, it’s possible to incapacitate yourself with a stun gun, if you come into direct contact with the victim’s skin. Generally speaking, this isn’t much of an issue, but if you’re using your knuckles as the point of contact, there’s going to be more contact with the victim. Given the character’s in powered armor it would be reasonable to assume there’s some mechanics to protect against unwanted electrical conduction across the armor’s shell. But, if there isn’t, then the knuckle becomes a very poor point of contact.
Depending on the design, it would probably be smarter to mount a retractable stun prod into the armor’s wrist. Worst case, this could probably be affixed with nylon straps and Velcro or even just duct tape, depending on their level of technical sophistication.