To start with, the definition of an allegory is that it is a type of writing that has a double meaning. On one level, it is a romance or adventure etc while on another level, it is a description of a moral, spiritual or political reality common to all people either actually or potentially. While it is not specifically stated in the definition of allegory that the characters have titles as names , it is a common characteristic of allegory that they often do substitute names
Having said this, the genre that Ben Jonson's The Alchemist is analyzed under is that of farce. Critics consider that his characters, which are similar to the types in Chaucer's The Canterbury Tales, are farcical rather than allegorical. Jonson is using farce, with a whole catalog of "typical" characters, to mock the social element of swindlers and victims, a prevalent aspect of Jacobean society.