I'm not sure what "deceivingly good" is supposed to convey here. Numbers that seemingly look good, but are kinda not? Numbers that are good, but you have to dig into them?
He had a 0.5 fWAR, 4.70 xFIP, 4.21 ERA. His GB% was below his career norms. His LOB% was below career norms. BABIP was slightly above career norms. K rate was average, but bb rate was higher.
When you did into his statcast numbers:
93rd percentile average exit velocity
95th percentile hard hit %
90th percentile xSLG
90th percentile Barrel %
7th percentile bb %
6th percentile chase rate
Whiff rate on his slider dropped 10 points. Put Away % on his slider dropped 12 points.
He got much more vertical movement on his slider, but it seemed to have caused players to recognize it better and lay off of it. At times, that pitch stayed well out of the zone. His slider used to be his out pitch, but it is much less effective now with a run value of -1. In 18 and 19, it had a run value of -14 and -11.
What does it all mean? IMO, Ottavino was ok. It was a fine season for him, but I think he's overpaid at his current salary. Batters are laying off his slider and no longer chasing it which is driving up his bb rate. If that slider is less effective, he's no longer a high leverage reliever.