Oh Petina! This little vignette dream does not follow the ongoing story/ dream arc in my Equestria dreamscape. I chalk it up to watching "Family Appreciation Day" and my subconscious wanting to dress up Petina Pony in a Playboy Bunny Suit. Only Petina could take a sweet, little charming ritual and take it completely out of context. She is really quite amazing that way. Sometimes she seems to be really on the ball and capable of seeing the big picture, and other times she the most clueless mouse/ pony in world. This is seems to be one of those cases.
Of course Applejack is suspicious of Petina anyways and considers her rather corrupting influence to young colts and fillies. So she wasn't too keen of the white unicorn mare showing up in such provocative attire. Applejack wasn't certain about all the specifics of Petina's little get up, but she's a smart farm pony and she can put one and one together and get two. For her part Petina was truly sad that she could not partake in the Zap Apple harvest ritual. In case you don't know what I am talking about here is a bit of info. [link]
One thing that Petina does enjoy about being a unicorn is that it is a lot easier to do things like tying and cinching things behind your back via levitation. Like the corset you're wearing.