I have one. It doesn't look sloppy and no one knows that I'm wearing one. Mind you, I do wear a stock tie over the top. I know several other larger ladies who also wear one as it's next to impossible to find a ratcatcher top in our sizes.
Here I am at a CDI-W showing at a pretty high level. It was hot as blazes and I had my coat off in the warmup. I would NOT wear it if the coats had been waived, just gone in my short sleeved shirt. Personally, I can wear a coat for the length of a test and rarely go without one.
I've got one for the schooling shows I plan on doing at some point. Larger girl here with a very large chest. There aren't any ratcatcher/show shirts that will fit me without looking horrible and tight enough to affect my movement and riding. I managed to find a coat (going up 3 dress sizes than normal) to fit my chest. But no shirts. Breeches are never a problem. Just shirts.
I think they just look silly, honestly. Shows aren't very long, and if it's honestly too hot jackets won't be required.
But really, unless someone takes their jacket off in front of you, how do you even know that they're wearing one? I'd much prefer to wear a show bib/dicky and wear a singlet under my jacket than wear just a shirt as I think that wearing a jacket looks much neater (especially as my jacket hides just how much of a fatty I am, LOL). I'd also prefer to look silly in some people's opinion than wear a shirt that doesn't fit.