The Pirates have improved under Don Kelly. They are still a frustrating team to watch, and need better bats and a better backend of the bullpen.
Don Kelly's record as the Pirates manager at the time of this writing is 11 and 14 which is a .440 win percentage. That is a great improvement compared to Shelton's win percentage during his time with Pirates at .410, and his win percentage just this year as the Buccos skipper coming in at .315. That alone tells you that the Pirates made the right move.
Don Kelly's first game as the Pirates skipper was May 9th, and ended in a 3-2 victory. Just looking at the below images you can't see how much better the team has done under Kelly. The surface level metrics look closer between the two managers than different. One thing you can see is that under Kelly the Pirates are performing closer to their expected numbers than under shelton.
Bucs under Don Kelly
Bucs under Derek Shelton
Don Kelly is finding ways to win rather than surrendering. It makes the Bucs watchable to an extent. 11 of Kelly's games have been decided by 1 run which is 44%. Shelton coached 12 one run games which is 31.5% of his games.