Yes, I would absolutely say that a healer who doesn't even attempt to put out any dps in the vast majority of content in this game is playing their job poorly. Just as I would say about anyone who is ignoring a bunch of their skills regardless of role. If the healer is fresh and doesn't yet feel comfortable doing so (new to the instance and doesn't know where the damage spikes are or just a new player overall in low level content) that's understandable but with time and practice anyone can do it just fine. It's one thing to be new, adjusting to the learning curve and improving as you go, and an entirely different thing to be purposely stagnant, refusing to use the tools this game gives us in the name of convenience so you can pay more attention to your second screen. Attitude is key.

I'm not sure why you are asking me about Cleric Stance use since that should be a given to turn on before putting out damage and off before going back to healing. Damage skills without Cleric Stance are pitiful just as heals with it are. Even if someone derps it's very fast to switch out, throw them a regen, and switch back.

I would expect healers to cover for the mistakes of others within reason. If someone is repeatedly being dumb getting themselves killed over and over even after being told to be more careful then they can eat all the dirt in the world for all I care. I won't burn all my mp on a deadweight in the middle of a fight. And if they are deliberately standing in avoidable stuff and trolling us they will get a kick. Non-fatal occurrences such as getting hit with aoe that you tried to avoid but were just on the very edge of (which latency can also affect) I just heal quickly and move on. I would be doing the same sort of thing to cover others' mistakes with any role so long as they are not a recurring problem and causing multiple wipes. For example if I'm tanking and someone accidentally gets proximity aggro on something I haven't pulled (like mobs on a patrol route) then I will grab it and adjust as needed, but if they are deliberately running way away from me or pulling groups with skills after I tell them not to (I don't main tanking but I'm more than comfortable enough with them all to manage large pulls without anyone pulling for me, it just makes things messier.) then I will gladly let them die and give them the boot.