Basically the title sums it all. People only like SA representation when the victim is perfectly innocent and never does anything wrong, aka, they don't cope, only accept the abuse and be pretty and are never sexual.  
Angel Dust is a drug addict, which is a very realistic and common coping mechanism for sex workers. He drinks. He copes by oversexualizing himself as a way to regain control over his body/life. And despite all, he's still capable of being kind. But people don't want the dirty ugly parts, they just want to baby the character.   
Example : Anya from Mouthwashing is never allowed to be sexualized in fanart. If you're an artist who dare to draw her as anything but an owo poor baby girl, you're a rape apologist or have a fetish or something, Idk. Some people are purist. Sex is evil. If you've been raped before, then you're never allowed to be sexual or like sex ever again. 

Some others claim Angel is a fetish character, and is wildely regarded as such because of the many instances he suffered on and off screen, but you know who else randomly gets raped and suffers in all of the duration for his own show? Ash Lynx from Banana Fish. But this one is regarded as good SA representation. You know why? Because Ash is a perfect victim. He doesn't exhibit any bad coping mechanisms (or any, for that matter, as far as I remember), is extremely high functioning and doesn't exhibit any PTSD behavior that affects his life or those around him. His trauma is beautiful. Real life trauma and traumatic response is UGLY. It ruins people's life, both the victim and those around him.

IMO Angel Dust is one of the best representation i had seen out there, even faaaar better than works like Banan Fish. He's imperfect, but that's what makes him good. 

People say Angel Dust is a terrible SA representation just because he's not a perfect victim