Have you tried every character? Since you use Fox, it isn't a surprise this can't seem to be escaped (due to his grav). I don't see this working on someone who is especially floaty and isn't a heavyweight but, what do I know?
I'm not sure how you could nerf it if it is really a problem. If you try to give Usmash less KBG, it'll just combo into itself even better at higher percents. If you try to give it MORE KBG, it'll kill even earlier than it does now. If you try to nerf his grab, it ruins his options by a lot, and Olimars NEED his grab afaik.
Wait, can't Uair be SDI'd out of? I KNOW you can SDI out of that! Especially when it's the White Pikmin, Blue Pikmin, and Fire Pikmin. There is no way you should be getting hit by the last hit of the Uair if you're SDI'ing up like Pikachu's Dsmash. Unless it's the Electric Pikmin, I can't see Uair working at all. Though, then again, you're Fox so... it's probably harder to SDI. But I do know you can escape his Uair pretty effortlessly depending on the Pikmin that's out doing it.
I don't really know a way to nerf it, if it is a TRUE combo on half the cast. shanus (being an Olimar main) or AndyG would probably be one of the two to ask about the combo, but shanus is currently away until Sunday.