When I have a creature with +1/+1 counters and I want to play a card with mutate and decide to put it on top of the creature with the counters, what happens to the counters ?
The resolving mutating creature spell doesn't enter the battlefield. It simply makes the creature that was already on the battlefield change characteristics. If that original creature was tapped, the new merged creature is tapped. If it had any counters on it, or Auras or Equipment attached to it, it still does.[...]
will they pump up the top creature ?
https://magic.wizards.com/en/articles/archive/feature/ikoria-lair-behemoths-mechanics-2020-04-02
Former Rules Advisor
"Everything's better with pirates." - Lodge
(The Gamers: Dorkness Rising)
"Any sufficiently analyzed magic is indistinguishable from science."
(Girl Genius - Fairy Tale Theater Break - Cinderella, end of volume 8)