The top of Doi Inthanon can get cold - below zero. While you're unlucky to die from hypothermia, to avoid a little discomfort packing a sweater and a pair pants can avoid a little frost nip.
During the wet season the mountain can also get alot of rain. Again, a raincoat or umbrella will no doubt increase the enjoyment of being on the summit.
Of course if you forget both the above, you can always sip an ice cold Coke in any of the many drinking/eating spots on the way.