I agree with "?" but will also say that the way in which your question is worded suggests that she was in the WWE initially and then went into the UFC afterward, which obviously isn't the case.
It's unfortunate when athletes who have been in other methods of competitive fighting who then go onto careers in the WWE (formerly WWF) that their prior careers are then either called into question, or they're considered to be washed-up has-beens. I've never been too big a fan of Rousey, but going into the WWE was probably one of the smartest things she could have done at this point: she'll likely earn more money, she'll have more exposure than the niche MMA fan base, and her propensity for injury will be drastically reduced (not completely eliminated) due to the predetermined matches, all the while she can still train and maintain her current level of fitness. This isn't necessarily due to her being washed-up, but because, as "?" points out, she seemingly has her focus on fame and fortune rather than competition, but that's making a lot of assumptions about her motivations.
But back to your question: did she bite off more than she could chew when she entered the UFC? I don't think she'd be nearly as popular as she is if she had, right, wrong, or indifferent.