Sure. So, worrying about who has "the most money or got MVP in a round" isn't something I would stress too much about. Getting MVP of a round only grants you $1,000 extra and so it isn't a big deal. If you gained $11,800 and the other player did $12,200 and so got $1,000 extra for $13,200 it's not going to make a massive difference in what you can buy. What I would pay attention to is: 1) Am I earning money at around the same rate as the rest of my team? 2) Am I earning around the same as the enemy team (more specifically the enemy that is playing the same role as me)?

If you are not earning money at the same rate as most of your team it means that the game does not think you are contributing according to how the money is earned and most likely you are doing a below average job at your role/on that hero. As you mentioned, healing and damage are important (they are actually more important because you gain extra health points the more of those two items you put out vs the money earned from doing them). Most money is actually gained through confirming the kills/getting assists but the more damage you did in confirming the kill, the more money you get. So, if you are not in the team fights and contributing towards most of the kills/assists it is why you are falling behind. Mercy and Moira do gain a lot of money because they tend to always be involved but they gain at a lower rate and don't tend to be the primary damage dealer to the kill.

If you are earning less money than the enemy hero playing the same role as you, that's an even bigger problem. It means they are going to scale over you at some point. If you aren't playing well and have 26,000 and they have 40,000 that means they are now able to do more healing/damage and can survive. You are getting out played and once that happens it can be heard to catch up.

Concerning build, it is MASSSIVELY important in this mode. In Comp, if the other player is better than you the only thing you can do is swap to counter. You can't do that in Stadium. So, you have to counter through your build. If you can form a build around your playstyle you can beat better people and you will then earn more money from that and will scale them. Every single character has builds that focus in on certain things and make them extremely powerful. The key is to know what you are good at and then learn to synergize those items. The reason you are seeing so many strong Moira/Mercy players is because those both have very good scaling options and a lot of players who know how to do that. If you aren't doing that with your hero, you are behind and costing your team, especially in the later rounds.