Peak holiday season in Dalaman runs between the early spring and autumn. In the midsummer temperatures reach into the high thirties and can be quite unbearable if you're planning on doing much more than lying on a beach or island hopping in a sailing boat. Even in March the temperatures are usually well into the twenties, and the sea is usually warm enough to swim in.
Whilst trying to take a beach holiday in the off season is not advisable, there is much more to see in this part of Turkey than just the beaches and the temperate climate means that it would be perfectly possible to take a hiking or sightseeing holiday here in the winter. You would also benefit from having the place completely to yourself, experiencing a more authentic slice of Turkish rural and village life and paying hugely reduced rates for accommodation.