That's a bit unfair. Echo was kinda pre-occupied this episode and Victor wasn't really a candidate for the abuse storyline and since Sierra is a regular, it gave them something to do with her. It has nothing to do with her being asian - all the Dolls are subservient and hot.
The "Random Kung-Fu dude" busted out the moves, but so did Ballard who isn't asian. Would you have the role rather gone to someone else just because they were afraid of it looking like they were stereotyping? Asians have a hard enough time getting cast in Hollywood as is. And really, everyone in Whedon's shows know Kung-Fu.
And as for "Chinese Restraunt Dude"...should he have have been Black or Mexican?
And finally, give the geeky girl some time. More will likely develop if Whedon's past shows are any indicator. I'd wager there is a lot more to her character than being Topher's lackie.
Put it this way: Patton Oswalt was fat. That's a stereotypical take on White people. What about the sex-crazed rapist? Don't all White guys care only about sex? My point being, you can apply stereotypes everywhere if you are looking for it. Why bother? How about people are just people and some will fall in line with stereotypes and some wont. That's just life.