Objectively how this game is designed a pure healer is an unskilled healer since they aren't able to use all of their toolkit to their advantage. Bad? Not necessarily, but like Bourne said it is definitely not more than average and personally I think that is being generous with just how much downtime there is to DPS on healer, in every kind of content from low level dungeons to savage, that I've seen in this game as a main healer since launch.
(Keep in mind this is all for normal situations where the tank isn't squishy and is actively using cooldowns; I can easily see if a healer has no time to DPS because of their gear being too low, the tank undergeared or a problem with the tank's play style and in that situation what I said above doesn't apply. Simply being unable to DPS since a party member is going to die is not a healer skill issue. Heal first, only DPS if no healing is necessary.)
Simply put your claim that it is "ridiculous" that someone is better than someone else is very telling that your thoughts on this are a bit skewed. I've been playing since 1.0 and subbed continuously to this day. I've raided on healer (WHM main 2.0-3.4 and then AST main from 3.4-now) and despite all my knowledge and skill there are players way better at AST and WHM than I am. I can heal and DPS in savage raids, but my numbers do not reach the level of other healers out there who are simply better than I am at planning their toolkit and remembering every little detail to push out as much powerful healing at the right times and all the DPS they can muster.
As an adult in general you should know that sometimes there are going to be people "better" than you are at something and it is something you really have to accept and it is not "totally unacceptable" since it is just the natural way of life.
Objectively this is how the game is designed unfortunately so despite what you've said a healer that uses their whole toolkit to contribute as much as possible is objectively better at playing their class than someone who doesn't...which is the same for every role and job. If someone as a pure healer can do 4000 HPS (but 0 DPS) and then another healer does 4000 HPS (but also contributed 1000 DPS), then which player is better? Objectively the one who contributed more is better at their class because they kept the party alive with the 4000 HPS but also did 1000 DPS at the same time which helped the group.
Now I am not gonna go into DF and call every healer I see who doesn't DPS "bad" to belittle them (in fact I say nothing to them), but when it comes to legit discussion on the facts of gameplay... it is a fact that if you ("general you" from this point onwards) use all your GCDs on healing in most cases you are playing sub-optimally and in some instances a high level of sub-optimal. There is simply way more you can contribute and you have more room to grow and be better skilled at the game if you reach for it.
Some people refuse to even try to reach for it. It would be a-ok in a solo game to play in such a way, but since in this game you co-op with many other players, all who end up having to rely on everyone in the group to complete content, if a player comes along playing extremely sub-optimal it may annoy the other players in the groups they get into and this is true for any role that is playing sub-optimally not just healers.
No one should go into DF and only do the bare minimum just because they can get away with it.