Are there any characters we know of (aside from Shrek, etc.) that has as much ADVERSITY as Krystal does? Let's check it out:
She's got the furry controversy, she's got the controversy about "she ruined muh Star Fox franchise!!1!", she's got the people who say "she'll get in the way of Wolf!!1!", she's got the general disliking due to past behavior of people fanatically supporting her from Brawl (and shoving her in lots of people's faces), she's got people complaining about her relevancy (especially because of SF: Zero), she's got the people who don't understand her potential because "she hasn't really done anything in the series at all!!1!", and probably a lot more I won't be able to recount on the spot.
Seriously, she's kinda like the most underdog-esque character suggested for Smash DLC excluding her popularity. There's a ton of depressing, ignorant negativity surrounding her.
NOTE: I'm NOT speaking about these as if they matter towards her inclusion; I'm speaking about these in the sense of the general adversity she receives.
Also, I'm sure that all other characters have their share of negativity. I'm just saying that their negativity seems to somewhat pale in comparison to Krystal's. It's probably because I pay more attention to the comments surrounding her than other characters, but still...
I'm probably a victim of the confirmation bias, aren't I?