Nami is surprisingly good at killing, but dont think that you're going to beat any assassins. She's very fun to play mid with, but unless you're very good at her, you're gonna be in for a rough time with ganks. Nami is pretty close to sona in terms of playing mid. Surprising burst, but awful at retreating, even with their speed boosts and cc.
Also just because a champ doesnt have high ratios doesnt mean shit. It's mostly about you personally and how you play. (Also, Leblanc and zed will shit on you, but hey, that's what happens when you pick a fun champ versus cheap players who like poorly designed champs.)