do i need to have my monsters at 100% motivation to achieve the highest payout in training? if not, what does motivation have got on showtimes

They should be at or near full motivation for the best stat returns. The game also suggests that monsters that consistently have high motivation at shows can produce children that have faster motivation gains. But as far as I know, no testing has been done to nail that down.

I'm not sure but my Pixie had about a 35% motovation and gained as much as my Dragon with about a 85% motovation. It might also have to do with the training policy and tiredness. But as i said i'm not sure.