Salty IS league average. Everything he does pretty well and everything he does pretty poorly average out to a reasonably normal starting catcher. If he can work on blocking pitches better and get on base a bit more consistently he'll be above average.
He's good for a 95 OPS+ which is actually substantially above the catching norm and he's a replacement level defender dragged down a bit in numbers by trying to handle the K-ball for the first time in his life last year. He also caught 31% of runners stealing against him, so his arm isn't awful. Like I said, it averages out. He compares roughly to a Miguel Olivo type, or did last year. That's not awesome, but it isn't a hole at catcher either.
Frankly before he ran out of gas in late August he'd been doing much better. September was murder for Salty and probably played a big role in what happened to the team. Through August he had a .781 OPS, which for a catcher is outright good. He'd been a nontrivial asset to the team up to that point. I'd expect him to try to improve his conditioning so he can take 400 at bats without flagging like that and that ought to make a big difference.