I do and I find it really helpful, in all matters of life. However, just to be clear, I don't exercise mindfulness when I trade, but rather in preparing for trade. I wake up around 6h30 and have my morning routine which includes a very calm breakfast outside, which helps me calm the mind and prepare for the day I have ahead of me. After that I usually take a relaxing bath and write an article (I publish pretty much everyday in the Brazilian version of Investing.com).
When it's time to get to trading (around 9 a.m. when the markets open here), I'm already pretty relaxed and shift into a highly focused state of mind. I then trade up until lunchtime (12h30), with occasional pauses in my concentration to, shall we say, "stretch the nerves". After that I focus on whatever other activities I have for the day, since I can no longer get in such a concentrated state for trading and try to exercise mindfulness then (that is, focusing on not thinking at all). Exercise also helps a load to keep your mind sharp and quick.