Why didn't Naruto help Nagato? When Nagato reanimated everyone who died in the village, Katsuyu was with him.

Couldn't Naruto use Katsuyu to help Nagato, or is it for some other reason?
When Naruto returned to Konoha, Tsunade ordered one of its smaller
  clones to stay in Naruto's pocket and keep him informed of Pain's
  abilities. naruto wikia


First and foremost, Tsunade didn't intend to heal Pain from the start but just wanted to located naruto instead. Even if Katsuyu implied to do, I think smaller clone are smaller in ability like healing and other. Probably, the Katsuyu in Naruto's pocket can't have enough chakra to heal Pain.


  Furthermore, Katsuyu's clones are extremely resistant to high amounts
  of damage, as seen during Invasion of Pain Arc. Katsuyu is also able
  to absorb people into its body, thus protecting them from physical
  impacts like the large-scale Shinra Tensei using its own high
  durability. naruto wikia


Aside from that , Katsuyu seem to absorb physical impacts only and it can't do nothing to chakra overexertion from as far as I watched.