South East Asia doesn't really have 'seasons'
Talk of weather in Thailand or in South East Asia in general is always weird to me. Sure it rains a little more in the so-called rainy 'season' and vice versa, but Thailand being the tropics is like any other tropical place in the world. It's hot. Always hot and humid. And then it'll rain once in a while, or it won't rain. That's it. That's the weather. Just have an umbrella. And when it pours, hang out somewhere for an hour or two and then the rain goes away. And then it's hot as hell again.
There's no real seasons to speak of. No where near the way we think of it here in the northern and southern latitudes with actual seasons. I've been to Thailand in August, and in December, and all year round, it was all the same to me. Hot and humid, and then it rained, and other days it didn't rain. There's no such thing as a 'cool' season anywhere in the tropics. If you think the difference between 90 degrees and 88 degrees from one night to the next is going to make any kind of difference when humidity is always around 85 percent, then you don't know real humidity.
So I say, go to Thailand whenever you want and can. Deal with the heat however you see best, prepare to sweat a bunch, and just enjoy the girls. Think of it like this, all the sweating and fucking you'll be doing, and the healthier food you'll be eating, you'll probably lose a bit of weight.