I think for a lot of people, and I count myself in this, is the length of time this all took place over. It was almost a year of abuse and pain that Star suffered.
Where a lot of us have a hard time simply giving FS the benefit of the doubt on DV and IQ grounds is 4 fold:
1 - it was over such a long time that ignoring the pain, frequent bruising, and obvious distress Star was in for that long is downright irresponsible and unfathomable, regardless of whether she has the lowest IQ imaginable, even a 9 year old would take their responsibilities for a child at least a little more appropriately and have a good degree more interest in their child's wellbeing. Whilst SB would've been an awful and manipulative person FS has frequently stated throughout this trial "my first priority was always Star", if that was truly the case at the time then Star was not just let down by her mum she was failed completely.
2 - FS did show an ability to tell right from wrong in relation t the treatment of her daughter, whilst SB had her under a spell for the most part FS showed an ability to recognise when SB was doing something wrong at times. FS stated she'd often lie to SB about putting Star on the wall as she knew it was wrong, FS stated that she'd send pictures of her being firm with Star just for SB and then wouldn't ever continue that. In other words she admitted to knowing at the time at least some of what SB was doing was wrong, that at least some of her behaviour was cruel/damaging, yet she still continued on with not just seeing SB but actively putting Star in her care regularly.
3 - The amount of times she actively avoided and lied to SS and to family and Drs in order to avoid Star being seen to / help being given. Yes on occasion SB would've played a big role in convincing her not to do this. However there was also several times where SB was not involved in the avoidance process. FS said she didn't want her daughter being taken away if they found something wrong with her so she just didn't get her the help she needed on those grounds. People reached out to her and tried numerous ways to help Star, and whilst SB definitely played a big role in FS avoiding some offers of help, FS had enough occasions whereby it was her own volition that cost Star valuable help.
4 - The evidence suggests she was knowingly physically abusive to Star at times too. The admittance that she "holds Star's head under water", the finger biting incident, and the supposed "choke slamming technique" were all physically abusive and grotesque actions knowingly undertaken by FS upon Star. Whilst she frequently denies she was ever physical, the evidence clearly suggests this was a lie, coupled with the countless other lies she is known to have told too it's easy to see how she's lying in that case too. Whilst she clearly wasn't anywhere near as physically abusive to Star as SB was, she knowingly participated in some physically abusive acts that she would've known were wrong and cruel at the time.
So whilst SB is clearly vile and the main reason for Star's death, and she deserves to be found guilty of murder, the reasons above are why a lot of us feel FS deserves a custodial sentence too. Whilst no doubt influenced by SB and struggled without much help for the most part, she had a duty of care to Star that even with her low intellect, she fell massively short of.