These flash events certainly make things worse. Some people are obviously in it for the rewards, with little interest for doing anything within the mode beyond the absolute minimum for progression. Most of these challenges are compromised through what is known in the sporting world as 'perverse incentives', essentially rewarding behaviour that undermines the spirit of the game.
I see people throwing rounds when they are leading so they can cram their 5 round wins into a single match. Same goes for maximising damage related challenges.
You can earn credit toward getting 2/5 matches played simply by quitting after legend selection. A 10 minute penalty is a pittance, and you could even say that leavers are doing a favour for all but those who actually wish to play the mode. And of course the earlier quitters accept their penalty the earlier their penalty expires, so if someone knows they are going to leave... the sooner they screw up the match the better for them personally.
Meanwhile the 'cause X amount of damage in a single match' challenges are so painfully low that any notion of them promoting good gameplay isn't even worth mentioning.