If you use a status move on a Pokemon with magic bounce, they will steal the move and use it back at you, so if you use Memento, will they end up killing themselves? If this is true, then Whimsicott could be a no.1 counter methinks.
No Memento is not reflected by Magic Bounce.
Memento is the only stat lowering status move that is not reflected by Magic Bounce.
Source: http://bulbapedia.bulbagarden.net/wiki/Memento_(move)
It's also not on Serebii's list of moves that Magic Bounce reflects back onto the target.
Hope I helped :)