I got more out of Rossellini’s performance on the second viewing. If you go in waiting for her big Oscar scene to happen, you’ll definitely be quite nonplussed, but she is quite a palpable presence throughout the film.
To be honest I think our perceptions have been warped by the routine category fraud these days where obvious lead or co-lead performances are shoved into Supporting. In the past you would get people like Beatrice Straight and Gloria Grahame winning for really brief appearances.
Just looking at the Supporting Actress nominees this century, these jump out as equally or less impactful than Rossellini’s:
Ruby Dee - American Gangster (Yes she has the slapping Denzel scene but it comes out of nowhere and for the rest of the movie she’s practically MUTE. And she almost won!)
Judi Dench - Shakespeare In Love (Even she acknowledges this win was a crock)
Jackie Weaver - Silver Linings Playbook (For me the most egregiously silly. I didn’t like that film much anyway, but she’s practically an extra!)