I'm not sure if you're referring to Medea as a character or the play itself, but if it is the play as a whole then the question gets kinda specific, and hard to answer correctly? But this is open to a lot of interpretation tho. As
Medea is a very specific type of play (tragedy), it conforms to special rules.
For
Medea's case the main tragic hero would be Jason, though Medea fits within some of the criteria as well, you can argue that the play challenges the notion of a "hero" in the Greek context in many ways (as stated below), good luck.

A "tragic hero" in Ancient Greek (or Aristotle's) terms refer to a character (often the lead) who possesses these characteristics:
1) A tragic flaw, or error (hamartia)
- This is fairly obvious for Jason for his excessive use of cold reasoning and callous decisions that starts off the play, and caused Medea's sorrow and grief. (You can describe Jason as Apollonian, which relates to the rational and ordered aspects of human nature)
- Medea can also be qualified as a tragic hero as she has a fatal flaw as well. As the antithesis of Jason in terms of gender roles and character, Medea possesses excessive passion and emotion. (It is also quite common for playwrights to portray women like this in Ancient Greece, they are often referred to as Dionysiac, which relates to the passionate and emotional aspects of nature)
- In a sense
Medea does not challenge the notion of a Greek hero, but conforms to the idea which has been passed down for centuries, when they made these kinds of plays.
2) Excessive pride (hubris)
- I think this is kinda self-explanatory, which yknow both Medea and Jason possesses, which is why they did not tolerate or condone each other's actions.
- This could be considered a hamartia of the "heroes" in the play (both Jason and Medea), and can be put in the same paragraph with the first point.
3) A reversal of events, brought about because of a hero's hamartia. (peripeteia)
- In this case the peripeteia here would be Medea's killings, which has caused Jason's downfall.
- Again Euripides conforms to the idea of a "tragic hero" and does not challenge it.
- The next point is where it gets tricky.
4) The discovery that the reversal of fortune was brought about by the hero's own hamartia. (anagnorisis)
- Essentially during the last agon both "heroes" attempt to blame each other for the events that have happened.
- You can say that both of them do acknowledge the fault of the other character in the play during its resolution, but they fail to realise that their hamartia counts as well.
- Here's where Euripides, and the play challenges the notion of a tragic hero, as the characters themselves do not realise their own flaws, with Medea being triumphant, and escaped the consequences of her reprehensible actions through divine intervention (a deus ex machina)
5) A character's fate must be greater than what he/she truly deserves.
- This is kinda subjective to interpretation as well, and it's kinda specific to Jason for the infanticide and all that fluff.
- You can say that he does not (?) deserve the punishment put forth by Medea, or you can argue otherwise as well.
- The gist of this point would depend on how you interpret the play itself, if you agree that he received far more than he deserves, then it is reasonable to state that the play conforms to the notion of being a hero, if not, you can argue otherwise.