Used to wear
an open, untucked shirt. Now have a vest.
Not what I wear in the real world, but the gun I use for IDPA also isn't my usual carry gun. I use a P90 Ruger for IDPA, and a Sig P232 for usual carry.
The idea (as I understand or interpret it) is to simulate situations. However, it is also a competition, a game if you will, so time and following specfic instructions (two rounds per target) are factors. In reality, I'm probably not just gonna stand there, I'm gonna be trying to move away from the threat. But in IDPA, you have to stay in the box, or move to the box, or whatever, to stay in competition. And you can't break the 180 rule, or you're DQ'd. And in IDPA (at least where I compete) there are sometimes steel plates that must be knocked down. My .45 will do that easily. The .380 must hit high on the steel to knock them down. So again, another failure to translate to reality.
But the main thing for me is to practice shooting at appropriate sized targets outside of the indoor range with the straight ahead paper targets. Where you can't draw from concealment and then shoot. IDPA isn't reality, but it's closer than some alternatives.
Long-winded answer, but some posters seemed (at least to me) to identify IDPA with reality. It isn't, so I don't worry that much that I don't dress the same for IDPA as I do for the rest of life.