My theory? The reason Rarity was so angry with AJ in that episode is because she doesn't want to accept the fact that she is interesting romantically in Applejack. Rarity was frustrated because she was jealous of Tremderhoof for getting to Applejack first, and she was worried that Applejack would reject her and be skeptical of her is Rarity did attempt a stab at romance.
I know that was not the actual point of the episode, but I like RariJack too much to pass up the opportunity.
she TECHNICALLY does love being covered in mud though. Also, that AJ squeegee grinding noise from her buttcheeks as she trotted away on the runway before turning around to give us her most glorious and professional duckface had my ribs ready to burst out my chest. I need to cut that scene out and make it loop. Or just copy paste it for a good minute long or something. Seriously, this whole episode had me groaning and laughing and just plain dieing with all the great jokes in it. Poor Spike, playing the friendzone is not how you'll win over Rarity. xD