We say the play is a drama because, well, you know…it's a play, a piece of literature that can only be fully appreciated when presented before a live audience. More specifically we dub it a tragedy. Of course, Euripides was a famous rule breaker and genre bender, so Medea has some big differences from most other Greek tragedies. The most significant of these is that Medea's hamartia doesn't cause her undoing.
The hamartia is most often called a tragic flaw, but the word is more accurately translated as a "mistake in judgment." A hero or heroine's hamartia typically causes their undoing. Usually they die, or mutilate themselves, or are at least driven insane with guilt. None of these things happen to Medea, however.
It's like Euripides sets his audience up for the expected downfall, then whisks the carpet out from under their feet right at the end. Medea definitely seems to have a hamartia. It's certainly a mistake in judgment to murder four people, including your own children, just because you got dumped. You could definitely say that Medea's passionate vengefulness is a character flaw. However, in the end, it's everybody else that suffers, not Medea.