There is a good deal of class conflict in Twelfth Night. This is only to be expected given the deep divisions that existed between the classes in Shakespeare's day. Whereas in real life such conflicts were generally kept in check by the authorities, Shakespeare lets them rise to the surface of his play, where he successfully exploits them for dramatic ends.
A particularly interpreting example of class conflict takes place between the notorious social climber Malvolio and the various servants with whom he comes into contact. Malvolio condescends to the servants, treating them like something he's just stepped on in the street. As far as he's concerned he's a considerable cut above the herd, which entitles him to look down his nose at those he regards as his social inferiors. At the same time, he looks up to his social superiors and positively fawns over them in a revoltingly sycophantic manner.
In a supreme example of class warfare, however, Maria, Olivia's lady-in-waiting, conspires with Sir Toby and others to play an enormous prank on Malvolio to make him think that Olivia is in love with him. This makes Malvolio even more stuck-up than usual, ensuring that his humiliating downfall, when it comes, will be all the more delicious. Despite being lower down the social ladder than Malvolio, Maria has inflicted a palpable hit upon him, a small, but significant victory in the endless round of conflict in this class-conscious society.