to Yellow Diamond, wearing the suit is almost like the punchline to a joke. its kind of a joke with the ponys who actually know a bit about her personality and values.
Yellow comes from a pretty wealthy family, and decided rather than live a easy life, she became a guard. unlike dilly and violet who are kind of forced to be guards, or shinning where being a guard is something that he slots well into, or cake who wants to climb up her caste in life and not end up farming apples her whole life in Appleoosa.

'Yellow Diamond is a great guard, and a pretty decent pony when she isn't being a complete bitch. she's the kind of mare who doesn't know when to stop, luckily in our case, that does extend to her work. I wasn't sure if any guard could make it in Las Pegasus, but I shouldn't be surprised that she could. She still wont tell me how she won that poker game against Mr. Horse.' — General President Apple Cake, 1010