No its fine. Flaring is good for bettas. I was worried about my boy Blue because he never flared, but when I put him next to Aqua,my female, he flared alot. But don't worry. He'll be fine.
In fact, if your betta doesn't flare alot during the day, it's encouraged to hold a mirror up to them several times a day for 10 minutes or so, so that they will flare.