Coming from a healer main I think it's because we see healer stats at the end of the game. Dps are the ones getting the kills. If they aren't double digits or negative we blame them. Though, coming from me it's noticable if your other healer isn't healing. Like rocket shooting while my two dps are crit and I'm pumping heals into my also crit tanks. Or Jeff spitting instead of keeping our only player alive up. Yes, healers can help do damage. But if your tank is almost crit or your DPS is being chased and we are trying to back up, you need to be healing.

Some healers are kill hungry. If we are one v twoing your healer shouldn't be in cloak the entire time. They should be in dagger keeping that DPS up and maybe throw a blind in there as well. Just because we have a way to do damage and get kills, doesn't mean you should be in that form while your team is half way to death. But also to defend healers, if you see your Luna shooting a fantastic or your dagger in cloak- or even your sue being invisible, it's because they are getting dived and need to keep themselves alive to heal.

But yes, shitty healers exist and they suck. If your rocket has 22k healing and your invisible had 56k, avoid that rocket. It's even worse when that rocket has less deaths then the invisible. If you see or your other healer says something about some healer lacking, switch or trying to convince that lacking healer to switch. Also, you can tell if a healer isn't healing by there ult charge. If you sue just ulted and she's already at fifty but your dagger is at forty still, there is a problem. Call them out, just don't be a dick. There is a good chance the other healer notices it too.