In addition to lines from Hamlet, Macbeth, King Lear, and Twelfth Night, Stephen quotes from Antony and Cleopatra in Proteus when he conceives of Mulligan as "fortune's knave." The allusion implies both resentment and contempt for worldly success.
In act 5 of Shakespeare's play Cleopatra has lost her lover Antony to suicide and must decide whether to follow him into the afterlife or to make her peace with the new sole ruler of the Mediterranean, Octavius Caesar. The decision is made for her when she learns that Octavius intends to remove her from Egypt and lead her in triumph through the streets of Rome. But even before she learns the extent of Caesar's political ruthlessness and his immunity to her charms, she readies herself to renounce life:
My desolation does begin to make
A better life. 'Tis paltry to be Caesar;
Not being Fortune, he's but Fortune's knave,
A minister of her will: and it is great
To do that thing that ends all other deeds,
Which shackles accidents and bolts up change,
Which sleeps, and never palates more the dung,
The beggar's nurse and Caesar's. (5.2.1-8)
In Shakespeare's iconography, Fortune is the amoral distributor of worldly goods and evils—arbitrary, inscrutable, and blind to human merit. Power, wealth, and status, Cleopatra reasons, amount to nothing more than "dung," and they do not say anything good about their possessor. He is Fortune's underling, her bitch, and thus "paltry."
This is, of course, the compensatory logic of someone who has just fallen from the top of Fortune's wheel and did not disdain its gifts when she possessed them. But it contributes to the magnificent rhetoric of transcendence that makes the end of Antony and Cleopatra a sublime experience. Stephen hitches himself to this wagon when he slights Mulligan's wealth, his assured self-possession, and his imminent worldly success as the trappings of mere "fortune." Like the political "pretenders" that he will go on to contemplate, he constructs Mulligan as outwardly splendid but inherently paltry.