Maria is indeed a stock character. Clever and cunning servants were a standard feature of Greek and Roman comedies, and they continued to be used in drama right up until Shakespeare's day and beyond. Such characters would often be portrayed as more intelligent, more clued-in, than their alleged social superiors. We see this with Maria when she fools the hapless, snobbish Malvolio with her remarkable skills as a forger.

But she's so much more than this. Maria is not just a cunning trickster; she's possessed of a rational intelligence that elevates her above the common run of stock characters. Maria is at the front and center of much that happens in the play. She is at the heart of the action, not, as would traditionally be the case with a stock character, on the periphery. Instead of reacting to events as would normally be the case, she shapes them, acting as a catalyst for a number of key plot developments.

How is Maria a stock character in Shakespeare's Twelfth Night?

Stock characters were first used in the Ancient Greek comedies, and since Shakespeare relied heavily on the classics for his inspiration, he derived the idea of using stock characters from the Greeks. A stock character is any type of character that is seen repeatedly within a literary genre (Dr. Wheeler, "Literary Terms and Definitions: S"). Common stock characters for Shakespeare were reckless young men, love interests, characters who create obstacles, the hero or heroine's parents or guardians, temperamental wives, soldiers who brag, as well as "outlaws, clever servants, female confidantes," and even a "jester, fool or buffoon" (Schwartz, "Shakespeare's Plays: Comedies"). From this list of types of stock characters, we can easily see that Maria fits the role of the "clever servant."

Maria's cleverness is especially demonstrated in the plan she comes up with to foil and humiliate Malvolio. We must remember that it was Maria who devised the plan rather than Sirs Toby or Andrew; the other characters merely followed along with her plan up to a point. One thing Maria finds really really irksome about Malvolio is that, not only is he very self-righteous to the point of being "puritanical," meaning excessively strict, he is also very loved and appreciated by everyone, especially Olivia. Maria describes him as being a "time-pleaser," meaning a "flatterer," and one who puts on airs of dignity (eNotes, II.iii.136). Since she believes that all love Malvolio due to his excessive righteousness and air of dignity, especially Olivia, her idea for revenge is to make Malvolio believe that Olivia sees him as her equal and has fallen in love with him, as we see Sir Toby state after she explains her plan, "He shall think, by the letters that thou wilt drop, that they come from my niece, and that she is in love with him" (II.iii.151-52).

Hence, since Maria comes up with a cunning plan to abuse Malvolio, we see that Maria fits the description of a "cunning servant" stock character.

How is Maria from Twelfth Night a stock character?

A stock character can be considered a stereotype, and in drama, a playwright employs stock characters to further the complications of the story.  Stock characters are used especially often in Comedies, as it is the recognizably stereotypical human foibles that make for good comic characters and situations -- foibles like craftiness, dimwittedness, miserliness, and  being crazy-in-love or a braggart.

Maria is an embodiment of the "crafty servant" stock character.  This character was found in Comedies of ancient Rome, but more contemporaneously to Shakespeare, the Commedia dell' Arte of Italy.  This comic and improvisational theatre form allowed both male and female performers onstage and might have been a very strong influence on Shakespeare's theatrical sensibilities.  The comic servant from Commedia who can be associated with Maria is Colombina, the crafty servant of the female lead of a play, a servant who also often had a love interest.

For more on stock characters, Commedia, and Maria, please follow the links below.