The way that snarky comments can hurt the community is by hurting people's feelings and making people feel reluctant to show off what they're up to. "Show HN" posts in particular are one of the coolest things about HN, and if people feel like they're going to get torn open just for putting themselves out there, they are unlikely to do so.
So there's nothing intrinsically wrong about snarky comments. It's when the snarky comments are targeted at individuals undeserving of such harshness that there's a problem.
If someone makes a "Show HN" post, and you think their work is incompetent, or that the problem they're solving isn't important, that doesn't excuse being snarky at them.
If, on the other hand, they have blatantly ripped off someone else's work, snarkiness would be fine (assuming you have damned good reason to believe that they did so intentionally, and that they did not ask permission).
We should be happy to make people worry about unethical behavior here before posting, because the ethics of your behavior can usually be assessed by introspection. We should not make people think twice about their work being incompetent before posting, because competence cannot easily be judged through introspection. Only by actually posting and seeing constructive feedback will someone be able to know how they can improve.
Of course, this applies to more than Show HN posts; people who read HN can also have their work posted by other people. The point is that we don't really need to worry about hurting Apple's feelings over blatantly ripping off someone else's work.