for doing absolutely NOTHING in the film.
Except that she made Harper Lee IMO, an equally fascinating character as was Capote. I would have loved to have seen more of their relationship, but the film was rightfully focussing on Capote and his work and Keener's was a support role, but a very well deserved nomination.

I LOVE Catherine Keener but I would agree. It is also possible that it was a weak year in the supporting actress category. I also love Helena Bonham Carter but her nomination for The King's Speech was equally not Oscar-worthy.

Very true, OP. Proves the rule at the Oscars that actors can be nominated just for being involved in an Oscar-worthy film. See it over and over.
I simply am not there

I agree with OP. I'm a fan of her in general but I don't see this performance as anything more than competent. Perfectly respectable but unremarkable.

I thought she brought the quiet power of Harper Lee to the role. My favorite scene is when a man inquires about Mockingbird. She confirms everything with a 'yes' in an effort to accelerate - and terminate - the conversation, which I thought was very true to the author's personality. She wasn't into hobnobbing like Capote was.

Yeah she sucks

Why wasn't Clifton Collins nominated for best supporting actor? That's what I cannot understand.