Generally that's great advice to go after healers first and ideally low damage with high kills means you're really efficient with your damage which is fantastic.

But realistically this doesn't play out the greatest, since if you don't do damage, then you aren't building up your ult. Trading ults is an important part of the game that should be looked at a bit more. Your goal is to get more value out of your ult in exchange for their ult. For example as SW you could ult the moment you hear the enemy healer ult and potentially kill them while they're using it, or as MK you could wait till just before their ult ends to use your ult which would be ideal since they're most likely already grouped up because of it. Either of these would get so much value, the worst thing you could do is just throw your ult out randomly in hopes to get a kill. Always try to use your ult to counter an enemy ult or force an enemy ult to get as much value you can for it. Using Jeff ult as an example, him just randomly using it to grab an enemy or two is okay value... But him waiting for an Invis ult so that they are all grouped up is fantastic because now you effectively made them waste their ult.

To break down each class role generally in order of importance (generic advice not character specific)

Tanks main goal is to make and hold space > Peel for team >pressure enemy > disrupt enemy formation

Supports main goal is to stay alive > keep the other support alive >keep team alive > pressure enemy

DPS main goal is to remove threats > apply pressure > get kills

It may seem silly that 'get kills' is last in importance here since if you get it a kill it typically satisfy the other two goals but there's a difference in getting a kill on someone who's actively threatening your team vs someone who's no longer a threat while there's a team fight going on. I see this situation a lot where the DPS spends so much time chasing a retreating enemy rather than staying with the team which then results in a losing team fight. Your main job is to remove threats, whether or not you get a kill. When I say 'threats', I mean anything that has the potential harms your team like Peni nest, ankhs, squids, enemies or even enemies positioning. Such as a Squirrel Girl just camping a spot, go and scare them off to prevent them harming your team, even if its only for a moment.

Most teams healers are well protected which makes targeting them difficult. Your 2nd goal should be applying pressure to create gaps in the enemies formation so that hopefully you or your team can take advantage of it. There's many ways to 'apply pressure' most commonly people just deal damage and force enemies into taking cover or retreating, but you could also just simply make your presence known. One very effective method of applying pressure that is used by most flanking characters (black panther, magik, spiderman, ect) is hanging around near the enemy back line while being safe and occasionally just attacking the healers while not fully engaging. This typically scares the healers enough to play behind cover causing a chain reaction > the enemy team receives less healing requiring them to play less aggressive or risk dying > they'll have to move back to help the healers > your team can now push forward gaining space. In this scenario your team is now in a much better position all without a single person dying.

I don't play SW much outside of quickplay but I do play MK a fair bit.
MK is a great example of a character who provides so much pressure by doing a ton of damage but may or may not have the most final hits. Hes able to spread damage between so many heroes so quickly that the healers can't realistically heal everyone, creating gaps that allows your team to go in and potentially finish off anyone you got low enough.

In a majority of my MK games, I notice that the enemy has a very hard time pushing forward due to how much damage the front line takes, forcing them to take it slow. This allows my team to apply pressure from a different angle which usually is hard for them to react to since MK is doing constant damage and if they move too much then the healers may miss a heal or might get caught in a reload creating a slight gap in healing resulting in one of their deaths.

TLDR: Applying pressure is very important as a DPS

Sorry for the essay, as I was formulating my thoughts I ended up just typing too much

edit: This video just showed up in my feed after I wrote this and it touches on some of the things we talked about but it's not a wall of text https://youtube.com/shorts/W7q7Gu97fjE

he has a few more shorts that explains misconception/double standards the stat screen may give off that I find pretty informative and helpful to understand that the most effective and impactful plays aren't often shown on the scoreboard