I want to criticize my girlfriends artwork
Why would you criticize her artwork?
I want to motivate her to improve.
That's good that you want respectfully offer your honest opinion of your girlfriend's artwork. Your insights could intensify her focus and improve her art.
Whatever I can do to make her improve I will do. If that means making fun of her then so be it.
You shouldn't be making fun of her artwork, I'm sure she's trying really hard. Instead you should give her constructive criticism on how she could improve.