I had a friend that showed her mini mare at open shows/EQ team shows with her maybe 4-6 months along, she always looked good pregnant or not though! She just looked a little out of shape.
But, then again you have people who show geldings that look like they are about to give birth!
(we have a morgan gelding who is 16 hands and 1300 lbs. He trims up nicely around 1150-1200.
But, when we first bring him out of the pasture in the spring, he's quite large! And, I had to show him like that before. We were in the ribbons in hand and riding classes alike!
I would show a pregnant mare, but that's because I go to fun shows, not rated shows that have huge entry fees where I would want to win. I'm going to be breeding my quarter horse mare to keep her out of heat during show season (and to get a nice little reiner baby that I've always wanted!) so she'll be prego.