I'm not an enormous fan of immobility in general, usually preferring when they're unrealistically huge but also implausibly somehow still mobile. If I had to think of situations where I do find it hot, it would be less of the "this character is just so fat they can no longer walk" but more situations where their immobility is only temporary. Something like they ate so much they're pinned to the ground by their overstuffed gut, extreme breast/ass expansion, muscles getting so insanely pumped up that can't bend their limbs...