It doesn't matter unless it's high-end farm content where they expect you to not be letting that happen (and it has been a problem occasionally in my time playing, where there's a healer in clear or farm parties that isn't healing enough and the party gets frustrated).
However, if a healer is doing that in, say, a dungeon, there is always the possibility they will call it out. People don't like the damage down either.
Usually when someone has died it's not been due to me doing DPS but people do like to blame it on the healer doing that anyway. And it can be possible they are right in certain cases because, although rare, there are dungeons and raids both that actually have intense healing moments, even if they get drowned out by the volume of content that only requires an odd oGCD heal.
I think there are also cases (including recently) where I've had to heal people due to them making lots of mistakes. And doing that means you're losing out on your damage dealing and "underperforming" by the metrics people care about in this game. So healers having the lowest damage contribution helps with convincing yourself to just patch up all their mistakes because you're probably at the bottom of these metrics whatever you do.
That's obvious to people with a rounded experience of the game. That sort of sentence is aimed at people who are new to healing or haven't played all the roles (properly) in the game and are spamming cure to get people to 100% HP.