Hi, I'm the anon who knows rose irl. She insults Bellatrix's looks because she's jealous. She's always been bullied for being fat. Her belly hangs down to her knees. She's really fat and short.
Oh God! I don't like body shaming anyone, even someone like Rose, but that does explain why she's so jealous of Bella and why she places so much value on male attention. The worst insults she can make up about Bella are Rodolphus and Voldemort not caring about her (which is objectively false according to the books themselves). It also explains why she insults her for being tall and slim and having a strong jaw, which is something only slim people can have.
It's really pathetic. I hope she gets therapy. She needs it badly. It would be far easier for her to just lose weight instead of spending her emotional energy on this.