It's unfair. Nobody, no matter what their size, should get bashed. But I find this very hypocritical as you said the woman in the picture's body was "really ugly". You are just as bad in that sense.
I didn't bash, I just expressed my opinion. And I've seen a ton more skinny-shaming than I've seen fat-shaming, so even if I bashed, I don't think it would be that unfair.
What you said was rude, no matter how you put it. And it's not a competition of who has seen more of what, more people are more accepting of skinny people than fat people. Saying someone's art and a body size as being "ugly" is extremely rude and offensive.
The character IS the drawing. And it's still offensive to call the character ugly. You wouldn't like it if someone called a character you drew ugly, surely?