Who should 6* Mega mastery tickets be used on, generally?
I use mastery tickets on mutants since Norns are easier to come by than mkraan shards.
Paywall or heroes or are difficult to farm.
I would use it for whichever character uses norns that you have the fewest of. In my case, I have a ton of blue and purple so I'd use it for green, red, or shards. Norns are easy enough to get that it doesn't make a huge difference.