is Mercutio's tragic flaw brashness?
It certainly could be argued that Mercutio is brash to a fault, though his dying words, "a plague o' both your houses!" paints him as a victim of the tragic feud between the Montagues and Capulets. At the beginning of Act III, Scene 1, the scene featuring his and Tybalt's death, Benvolio urges him to "retire," claiming that in the heat of the day, there is "mad blood stirring," and a fight with the Capulets seemed imminent. Mercutio dismisses his fears, though they turn out to be well-founded. When he encounters Tybalt, he treats him with aggression and disrespect, attempting to goad him into a duel. Later, when the newly-married Romeo refuses to fight with Tybalt (now, unbeknownst to everyone, his cousin by marriage) Mercutio brands his friend a coward, accusing him of "calm, dishonourable, vile submission." He then draws his rapier to fight Tybalt, and is ultimately killed as Romeo tries to break up the duel. So Mercutio is certainly excessively bold in his final encounter with Tybalt.