There are situations where you're guaranteed DPS time and situations where DPSing might put your party in danger. The trick to being a top notch healer lies in identifying these situations from experience and maximising your contribution to a party.
A healer is not a bad healer for just healing but they're a bad player if they are otherwise stood scratching their arse. Healers are given Cleric's Stance and an arsenal of offensive spells for a reason. That reason is not for smoother solo play. A lot of dungeons are designed around monsters not annihilating your tanks in seconds. There are plenty of moments where you'll find time to yourself to stance dance and throw a few DoTs or nukes around before returning to your healing duties.
Doing the bare minimum of your job is what I would consider being a bad player. You could be good at the bare minimum; I have met plenty of healers who never let people die but they don't do much else. Yes, they are good healers but they're not good players. A good player plays to the best of their job's ability whilst maintaining their key responsibilities.
You could say this is merely a debate of semantics since people have varying degrees of good and bad. What most people seem to be saying here is not that somebody is a good or bad healer if they do or do not nuke. They're saying somebody is a bad player if all they do is heal when they have time to nuke or people die because their healer is too busy nuking.
That said, it happens with every job. Some people are okay with doing the bare minimum. Others make the effort to be the best at their job and take advantage of every mechanic possible to make the group's progress more efficient. Dragoons who use Mantra when there's no Monk for big hits, tanks that swap between Sword and Shield Oath depending on their duty, etc. There are many examples that we could use to label people on other jobs as 'good' or 'bad', but I would argue such players are good or bad players, and not good or bad Dragoons/Paladins/etc.
Same for healers. If a healer just nukes or heals I would call them a bad and lazy player, not a bad healer.