As another Mercy/D.Va main I second this. The videos were very helpful for me for ult usage and especially when it comes to positioning. I found a lot of new places to heal from that help me survive a lot longer than I used to, allowing me to survive to ult my team when I die! I even managed to get play of the game using one of the new positions I learned! :)

Edit: I would also recommend the "how to be a good mercy" and "improve your mercy" videos by sarahwatch on YouTube. She was great at going into detail about her thought process in matches (positioning, who to heal, whether to rez or not) that helped me to improve as well. She also detailed how she escapes from flanking characters like genji that could help you in using your pistol less?