How often should I be using damage boost? Just basing my stats off of a lot of other Mercy players I see it seems like my damage amp stat is way too high while my healing isn’t nearly as high. Overbuff shows that my damage boost stat is extremely high as well while my healing is… average (but I’m not sure how reliable that site is). https://www.overbuff.com/players/pc/Bratty-11447?mode=competitive
Should I be focusing more on healing during a match? Maybe that’s why I can’t climb past 2550 
In Plat, Mercy is a hit-or-miss type of character to climb out of Plat. She is so team-related that typically based on your teammates’ performance will determine your outcome.
It is possible to climb out of Plat with Mercy as I have done it ages ago, but even then some, playing heroes who can heal, damage, etc. will be your best bet.
Or you could try and duo with a Tank or even DPS that way he or she will utilize your whole kit and not just damage boost.
But to answer your question, you want to typically damage boost more especially during Valk… What I do is I find that one teammate that typically is outperforming in terms of damaging and pocket them as they will get more usage out of my damage boost. I always said this, but the higher you climb the easier it gets to get teammates to utilize Mercy’s whole kit, but she herself gets harder to play in terms of survivability.
Hope that helps a little.
Mercy is a game sense hero. You should be using guardian angel – ON COOL DOWN! If you’re not bouncing around damage boosting the person doing the most damage at that moment in time, healing up who needs to be healed, and then immediately returning to damage boost the main damage dealer, you’re doing it wrong. Let me put it this way, there is no such thing as too much damage boosting unless you’re letting your team die. Healing should be the unwanted chore you have to sometimes do, not your main purpose or goal.
You should be helping anyone win a 1 v 1 via pocketing them. If you are not helping DPS win 1 v 1 matches, you’re also not doing it right. If you’re not helping to break shields, you’re not doing it right. Your actions per minute should be through the roof.
Of all the heroes in the game, I think Mercy is the most under utilized and one of the most misunderstood. If Hanzo is sitting there trying to break shields behind his friendly Rein, 9 out 10 Mercys will be there healing up the Rein who’s already at or mostly at full health, while totally ignoring the Hanzo because “he’s at full health, so my job here is done.” It’s actually mental.
There are 5 people doing stuff at any given time, you should be damage boosting the one who’s doing the most useful thing, and you only stop doing that when someone needs healing, then you go right back to your main task which is helping most your productive teammate succeed in what they’re currently doing - be that breaking shields, winning a 1 v 1, helping your main tank as he fights the other main tank, etc.
Technically mercy’s entire kit favors pocketing DPS so you should be damage boosting most of the game. Obviously if a tank or support is about to die and you can save them by healing then you should but that goes along with high game knowledge of knowing when someone’s going to die.
You should also look at ult Economy.
If somebody on your team is pretty close to his ult it might be a good idea to db them so that they get it before the next teamfight.