Choosing to only heal as a healer and ignore your DPS kit is exactly the same as a BLM choosing only to use blizzard spells and ignore their fire spells. Both are examples of trying to roleplay as a playstyle that does not exist in FFXIV. It is incredibly rude and disrespectful to waste other players' time because you want to pretend you're playing a different game. That is not how FFXIV works. If you do not like that, either find an RP party or play a different game.

That said, there's nothing wrong with not performing optimally as long as you are genuinely trying your best. No one begins as a savage tier player and it's okay to not be perfect. Additionally, if you have certain physical or mental limitations that make it harder to play the game, it's important to find a group of players who are willing to help you get through different aspects of content. Unfortunately, an MMO just isn't the prime example of a game genre known for accessibility, but as long as you communicate with people and create a network of friends who will help you, you can still enjoy a lot of what FFXIV can offer you.