She's quite something when it comes to putting herself down.
Literal
She self-deprecation thing [at-では] quite-something [copula-よ].
自分をけなすことではなかなかのもの — けなす means 'to belittle, put down.' なかなかのもの means 'quite impressive, not bad at all.' A ironic observation about someone skilled at self-deprecation.