Very nice shot and proof that the Tamron 90mm macro is a great portrait lens as well as a brilliant cost-effective true macro.
I really like the subtle backlight, which shows off the fur nicely.

I'm not so sure about any lightening of the shot, I feel it has a nice warmth. If I were to do anything to it, I would give it a crop down very close to the monkey's head. The sky (on second view, I realised it is not sky but soft focus jungle slopes) feels a bit imposing but to no good purpose, as there is no detail in it. So get rid of as much sky as possible.
This, for me, will put the little chap's expression in a much more dominant place in the overall frame. The shot will look more panoramic and gain some impact because of the unusual framing.