The Victorian Dandy is described as a bachelor which has the following characteristics:
a. Living above their means
b. Over- preoccupied with aesthetics and fashion rather than social responsibility
c. In Wilde's words "wake up near 3 in the afternoon, dine at 5, then the opera, and go to bed near 3 in the morning"
d. Surrounded by the likes, hating on anything ugly, completely oblivious of human emotion.
Algernon comprises ALL of those characteristics. Opulent, he has creditors running after him. Excessive, his hunger and eating habits represent the way he lives life: Excessively and always wanting more. Flamboyant, as he always expects the best champagne, the best restaurants, and always dresses to the part. Immoral, as he leads a double life and does not mind meddling in that of others. Fascinating, because he simply admits it and does not care.
Post Note: Oscar Wilde was a fanatic of the ideal of the Victorian dandy, and for this reason he ensured that there was a dandy in most of his works:
In The Picture of Dorian Gray, the dandy takes the shape of Lord Henry Wooton
In A Woman of No Importancethe dandy is Lord Illingworth.
In An Ideal Husband, the dandy is Lord Goring.
Hope this helps!
Much of the idea of being a "dandy" centers on creating something based on a false premise. Whether it is a false sense of importance based on creating perhaps a personal history that is grander than the real one or dressing a certain way and adopting a particular persona in order to gain attention, these can make a person a dandy.
Algernon's creation of Bunbury in order to allow him to always have the excuse or a way out of any kind of compromising situation mimics in some ways Jack's complete change of personality based on his current location. In this way both of them are acting like "dandies."