It's hard to think of Kira as an FO. Her character should have been much more T'Pol-esque. She never performed any of the typical FO duties. I can't ever remember her leading an away team, or credibly taking command in the Captain's absence and actually resolving a situation. She didn't act as an intermediary between the Captain and the rest of the crew. And she didn't even throw her weight around as the "Bajoran liason". There was a chance to make Kira much more of a challenge to Sisko in this respect (because of her backing from the Bajoran Government) but it never happened.
She led away teams a few times. I remember when her taking command of the Defiant to find out what the Klingon's were looking for. Let's face it though DS9 had far less away missions than any other Trek show.
She did take command of the Defiant when Sisko was unable to in Tears of the Prophets. Even over Worf! Then she was running the station at the start of season 7. Defying the Federation when the Romulans were putting weapons on a Bajoran moon.
And the Bajoran liaison stuff was brought up a few times. However when Sisko accepted the Emissary job well he didn't really need a liaison officer cos well he was part God to them.
I think though even if Kira wasn't a model FO she couldn't have been expected too. However she was hands on, capable of running mission. Had the respect of the crew. Easily able to take command and point out when she thought people were stupid.