When you really get down to it, despite Jonesy's laziness and immaturity, he's ultimately a much better candidate for a customer service job: he's a good-looking, charismatic, extraverted, has no visible piercings or tattoos, and can at least pretend to be a hard-worker.
By contrast, Nikki (though certainly not shy) is a lot more introverted and, compared to Jonesy, looks/acts like your typical rebellious tennager: she has a face full of metal piercings, is verybrutally honest (even more so than Jonesy can be), can (at times) be just as lazy as Jonesy is (and never really does anything to even try and pretend to be a hard-worker) and also isn't exactly the friendliest person ever. Overall, Nikki just isn't the best candidate for a customer service-job (which is basically almost any job within a shopping mall).
Also, remember how, in the pilot episode, when Jen was given the task of choosing her replacement at the Big Squeeze after being hired at the Penalty Box? She ultimately chose Caitlin as her replacement, despite the fact that Caitlin technically never applied for the job, since all the actual applicants were, at least to Jen, very weird-looking (one of them being a scary-looking Goth girl).
In "Employee of the Month", Chrissy temporarily brainwashes Nikki into becoming a girly Clone, making her act exactly like Kristen and Kirsten, only reverting back to her normal self with the aid of her friends. Kristen and Kirsten are never shown to have any friends other than each other and Chrissy. They rely on Chrissy to tell them what to wear, what to do, and even what to think. If Chrissy knows how to change someone's personality on this level without the use of torture instruments and/or major brain surgery, who says the other two Clones weren't brainwashed too, and just didn't have friends to snap them out of it?
Turns into Fridge Logic before rounding back into Fridge Horror when you consider this: Chrissy is the most independent of the three clones. She is, in fact, a Closet Geek and hides this from the other two. Then you remember that one of the factors in brainwashing Nikki was the "exclusive Employee of the Month lounge." Said lounge is actually a brainwashing facility using a form of hypnosis to enforce strict conformity and anti-individualism in a manner that would have George Orwell spinning in his grave. Does Khaki Barn brainwash all of their employees? Is Chrissy a Closet Geek because she doesn't want corporate to catch on and brainwash her?
As if the Orwellian brainwashing wasn't enough, it gets worse: it's offhandedly mentioned in a future episode that the Khaki Barn exploits sweatshop labor in overseas countries for its clothes. Who the hell is in charge of this corporation?