It makes a little more sense when you think about how when she was introduced in season 1, she was an Obstacle To Overcome in the romcom formula, rather than A Real Actual Character: She was Josh's bitchy girlfriend who had it out for Rebecca, our spunky heroine! And she was mean, for sure, but consider: She was (A) right about Rebecca trying to steal Josh, and (B) frustrated by her entire life. Her relationship with Josh was stagnant, she was unhappy with her career, and she had no friends outside of Josh's friendgroup.
In season 2, Rebecca befriends her (and remember, back in one of the earlier episodes, Valencia was happy to be friends Rebecca) by getting her out of her post breakup funk. And yeah, it seems a little weird, but it's like, Rebecca also helped her finally break out of her deadend relationship with Josh, so. I did think her planning their wedding was weird, but season 3 has Rebecca being all "You planned the wedding you wanted!!" and that sort of re-contextualized it a bit.
I dunno, I guess it just seems like Valencia fits into the show's theme of perception. Compare her to Paula, who was first introduced as the Office Bitch Who Was An Obstacle, but then by the end of the first episode is upgraded to The Sidekick, but then later the show reminds us she's still The Office Bitch to her coworkers no matter how kind and supportive she is of Rebecca (or even more directly, remember that scene where she goes ballistic on her coworkers for not refilling the ice tray? We know she's actually angry about her husband's infidelity, but she's still taking it out on her coworkers). Consider Nathaniel, who was introduced as a huge asshole, but then falls in love with Rebecca and becomes a little nicer, but then we get stuff like him trying to have Josh's family members killed and deported and "He's the New Guy" to remind us, unsubtly, that he's still an asshole.