Termie,
Indeed she plays a small role, but I don't think that is a reason to deny her an award.
The award is for what she brings to the character as an actress, whatever the place of the character in the whole story. Its not really meant to award people for "clinching" an important role/part in a movie but for good raw acting. When an actress can bring heart and memorability to an unimportant character, that's even more points to her, not less.
Supporting actress/actor is the award for all those non-main actors/actresses you seen in all films. If it were limited to only important roles, it would be an injustice. There would be no way to reward any of the other actors not in the main leads, no matter how good they were/performed. The focuss of the award really is to reward skill and talent, not other things beyond the control of the actress (or actor/writer/director etc)
I also think you are somewhat downplaying Patsy, She basically highlighted the dynamics between the Mistress, her Husband and the slavery system. I remember most her Mistress's mistreatment of her actually and her no-win situation much more than the rape and beating scenes, to be honest. I believed her misery, even the short departing scene with Solomon sticks to my mind. I genuinely kept wishing someone would rescue her from that place. The actress who played the mistress was also praised for bringing believability to her semi-good but still evil character. Plus word is, just as Steve McQueen did, Lupita (and Echiwetel) were VERY faithful to the actual story of Patsy and Solomon Northup as he told it. That I think is what earned her the critical acclaim, the small role may have played in her favour, not against her.