Now that my MAG is level 200, I see that he still gets hungry and has a drooping animation. Is there any benefit in feeding your MAG at level 200 or am I stuck having to feed my MAG just so that I don't have to see the battery and it's sad body language 
?
You need to keep its energy up if you want to make use of its support & action abilities. (Whatever these are named in the NA version.) Their abilities only activate when they have energy, as each ability cost a certain amount of energy. (Excluding Photon Blast cause you'll always have access to that, no matter their energy level.)
@ErinXh Thank you! As a side question it doesn't matter what you feed it after that? Maybe I can put monomates to good use.