The theme of the folly of ambition in Shakespeare's Twelfth Night is apparent in the character of Sir Andrew Aguecheeck who pretends to be Sir Toby's friend, but he really scams him for his money.  Not of the upperclass, he entertains aspirations of Olivia's love.  However, he proves himself little more than vain and cowardly. Probably, then, the best exemple of foolish ambition is in the character of Malvolio.  Because he is no Puritanical, dampening the anarchy of enjoyment that prevails with such characters as Sir Toby and Sir Andrew Aguecheek and others, Sir Toby and Maria devise a plot that exploits his desire for Olivia.  In Act III, Scene 4, as he enters in his yellow stockings and crossed garters that cut off his circulation--and denote derision, Malvolio presents a most ridiculous figure in his ambitious vanity.  Yet, he insults Maria by stating that he is superior to her, comparing himself to a nightinggale and her to the daw, a crow-like bird.  And, of course, his famous lines are leant much humourous irony in this scene:

'Some achieve greatness'

And some have greatness thrust upon them.'  3.4.40.43)

As Sir Toby and Maria feign sympathy for him when Olivia begins to doubt him, Malvolio disparages them,

Go hang youselves all!  You are idle shallow things:  I am not of your element.  You shall know more hereafter. 3.4.116-118)

Although his aspirations toward spirituality seem genuine, Malvolio's inflated ego and his elaborate fantasy about life bring him nothing but ridicule.