well, for me it's cause she totally murdered the girl stereotype. she's strong, a fighter, a tomboy, and is always willing to show up the guys. I love this so much because I'm also a tomboy and when I was younger absolutely hated it when someone expected me to be afraid of spiders and bugs or run off crying when some boys shoved me down (I actually beat up 2 older boys when I was 8 because they dared stereotype me, rather than they pushed me into the dirt and flung a spider in my hair).
I love her altogether, be it cub, adult, personality, whatever. besides, I don't think she's a let down as an adult because she got the other Outlanders to follow her in ending the war. plus I personally think the reason she was so obedient to Zira was that she was actually trying to prove herself just as good as her brothers, who's to say she never wanted to be the chosen one?