The game is actually designed to have both healers and tanks contribute to overall DPS. That's not just a personal playstyle expectation that others are trying to force on you.

Healing obviously takes priority, but healers have an abundance of instant healing alongside tanks who can go awhile without needing a full heal. If you're not spending the extra time on assisting with DPS, it slows the whole pace of the content down. That affects your group and can understandably cause a negative reaction.

In most casual content, people probably won't say anything or they might just roll their eyes at it and move along. But if you choose not to DPS, you'll have to accept that there will be people that notice and sometimes there will be people that are vocal about it.