OP asks about iconicity, which is not immediately affected by current sales but overall performance in the collective consciousness.
Mortal Kombat and its characters are more popular right now, yeah, but Chun-Li is still a globally recognizable character. Countless cameos, references, costumes; and misguided or not, there was a movie about her :p
Her distinguishable design helps, I think. Unlike Scorpion and Sub-Zero, where I'd be hard-pressed to remember their respective looks.
This is similar to that recent thread "Is Ellie more popular than Lara Croft now?", where yes Lara Croft is more iconic, but Ellie (From the Last of Us) surpassed her popularity atm.
A nice example used in that thread was Superman. Most iconic superhero, but he isn't that relevant right now. Still, he's the superhero by antonomasia. That's Chin-Li.