Overwatch Mercy – How Did This Mercy Get 104% Kill Participation?

What you see is what you get! It's just an opportunity for players in the match to commend each other on good gameplay. :)
_{@PlayOverwatch on Twitter (May 30, 2016)}

Cards can reach "Epic" or "Legendary" status at 5 or 10 votes respectively. The only 'reward' is a unique voice line from the hero on the card. There's a full list of commendation voice lines on the Overwatch Wiki.

In other words, commendation votes are like clapping. They do not have any long-term tangible impact.

McCree is great at medium to close range but not that good at medium to long range.

I treat McCrees almost like I do Reapers: get out of their close range high-damage zone and you'll be fine.

Genji is not a good counter for McCree*; use any hero that'll allow you to do decent damage from out of flashbang range.
Sniper heroes (Hanzo and Widowmaker) are good at this, Pharah can conveniently drop rockets from above on him, Junkrat can also keep him at bay with his traps. Of course turrets (either Torbjorn's or Bation's) are great to keep any hero away.

Every hero has something that they're great at and it will be hard to defeat them if you go up against their best game. In McCree's case, it is dealing damage at his flashbang range. Do your best to avoid that and you should be fine.

_{* Genji is not a good counter because he usually needs a combination of some of his Shurikens, melee and Swift Strike (occasionally two of these will suffice) to kill another hero fast enough. Genji doesn't do well with long battles against other heroes and this combo will most likely alert McCree to Genji's position and get Genji in flahsbang range before he can kill McCree. This is everything McCree wants and it'll force you into the terrible baiting battle you described.}