At the beginning of each end step, if a +1/+1 counter was placed on a permanent under your control this turn, put a +1/+1 counter on Fairgrounds Trumpeter.
Walking Ballista enters the battlefield with X +1/+1 counters on it.
If I bring in a new Walking Ballista - then it enters the game with the +1/+1 counters already on it. Technically you could interpret that as meaning that the +1/+1 counters were not placed on it that turn - they were already there as it entered the battlefield.
So does the trumpeter trumpet louder and the crowd cheer more when they see the ballista walk onto the field?
(Obviously if a new counter is added after the Ballista is on the field then it would, this is specifically about the replacement ability on entering the battlefield).