He doesn't make a ton of glaring mistakes but he is obviously having problems preventing shots against at least.
I certainly hopes he improves defensively and grows as a player. I'm not ready to just give up on him as some are...guy has barely played half a NHL season and as you say is still very young for a defenseman. I think there are two issues though...
One is that management and the coaching staff don't seem to be concerned with him bleeding shots like crazy and overall not actually being good defensively. They seem to see him as a complete all around player that they're willing to run out there again and again despite his shortcomings that need to be addressed. It's a real concern that his problems never will be worked on and fixed if the coaches aren't being honest about his on ice abilities. That's not good for him or the team.
Two is that another young quality player in DeAngelo seems to be getting the reverse treatment for no discernible reason. I don't want to make it a DeAngelo versus Pionk thing, I'd be happy if both of them could develop into good players for the Rangers, but one guy bleeds shots like a stuck pig but is considered a promising all around defenseman and the other does nothing but put up quality shot stats and generate scoring chances but is considered a risky offensive defenseman who can't defend and is benched for any small mistakes.
It highlights what could be a real issue with how this teams management and coaching staff evaluates players. If you want to just shrug that away, I would make the argument that it's exactly this kind of evaluation and thinking that leads to teams giving guys like Girardi and Staal long term contracts and playing them in important minutes at all times even though they suck statistically.