Since you'd never leave Mantine in on a physical Electric attack even with Mud Sport, isn't Amnesia superior to Mud Sport anyways?
I mean I wouldn't run either. Scald / Roost / Defog / Substitute would probably be my go-to set, but that last slot is kind of filler anyways so I could see Amnesia putting in at least some work there. The thing is, even with an Amnesia (or Mud Sport) boost, most of the OU-relevant Electric types (Manectric, Thundurus, Magnezone etc) still 2HKO you with Thunderbolt, and it of course does nothing to remedy its mediocre physical bulk. Plus defensive boosting moves in general are kind of meh.
The one advantage I can see to Mud Sport is it provides some measure of team support, I suppose. Maybe a niche option for teams that don't like fast Electrics, but even then it's just not nearly as reliable as a better check like Hippowdon or Yache Berry Landorus-T, because there's no fucking way you're switching Mantine in on Mega Manectric or Raikou AND setting up Mud Sport in their face.