Stats- and cost-wise she's Agent Coulson with a different ability. Agent Coulson will give you draw which is incredibly, whereas Marvel Girl lets her attack and thwart simultaneously. To go further into her ability, there are tons of minions in the game that have 2 scheme, meaning that for the purposes of thwarting the main scheme, attacking a minion will be simply more efficient. However, if you give her Mission Training, you'd be hard pressed to find a minion with 3 printed SCH, as there is a grand total of 8 in the whole game (one of which is from Print and Play set and another from a Nemesis set) at the time of writing this, making her ability harder to get good value of. A thing to consider is attacking minions with 1 HP left, which would produce value beyond just the sum of her ATK and the target's SCH. Also, keep in mind her ability is hard-countered by the Crisis Icon.

To compare further to Agent Coulson, they both provide good traits. Coulson is a Spy, so he allows you to play cards like Spycraft and Marvel Girl is a Psionic, which lets you use Cerebro's full deck search.

You could also try to give her Sidearm and/or (with help from another players) cards like Attack Training (instead of Mission Training) to make her a well-rounded workhorse.