In my extensive experience with BKK I can't recall ever having gotten a mosquito bite at the airport. You're transiting with no plans to go through immigration, correct? In lounges, gates, etc. I think you are extremely unlikely to be bitten.

There are some areas at the airport that might be a little more prone to mosquitoes, such as the Thai-style food court on the basement level, but it seems like you will be far away from those.

Of course there's no guarantees. If it were me and I had any concern I'd probably put some repellent on anyway just to be safe.