And I didn't say he was awful, either. But there's nothing about his playoff record, or wins, to indicate he's anything beyond mediocre in the postseason.
I say that not just because he's lost series, which is understandable, since, as you pointed out, the teams he did lose to some of those years were very, very good. But just how he's lost series - the Denver series included, which was likely the biggest choke job in Jazz playoff history.
However, the total lack of competitiveness in those series makes it impossible for me to grade him anything but mediocre at this time.
Okay, give him a pass on 2017 since the Warriors only lost one playoff game that year. But in 2018? Meh. The Rockets were definitely a good team, but the Jazz did no better than Minnesota, as an 8th seed, did. In 2019, the Rockets again easily beat the Jazz and were a pretty easy out for GS in the second round.
Again, not saying he's bad here. But there's nothing that says he hasn't been mediocre in the playoffs at this point. Some of it may be a result of unfortunate seeding, but as I said already, that's why my view may change this year. If Utah makes it to the WCF, I think it's clear that this is the case. But until that happens, I can only go off what has happened already and those results aren't all that great.
But for transparency sake, I think the franchise as a whole has been pretty damn mediocre in the playoffs - and that includes Sloan's tenure. The Jazz is the 4th winningest team in NBA history and something like only 19th when it comes to playoff win percentage. If the team overperforms, it's almost always in that 4/5 match-up, which is about the weakest you can get in overperforming - but have underperformed, or laid some serious playoff stinkers, far more than they've ever gone out and shocked the NBA world as a strong underdog (in fact, I can't think of the last series the Jazz entered serious underdogs and pulled out a win).