While having a better cleave does not make the swordmage overpowered, its a good indication that greenflame blade is overpowered. Since the swordmage and fighter have the same role, we can compare their powers directly.
Cleave's main strength is autokilling minions. Greenflame does the same thing, except even better. Since cleave isn't considered a weak power, then greenflame blade is a very strong, potentially overpowered power ESPECIALLY because it works just like cleave does and does it better.
Its one thing to have a good power that does a slightly different effect, its another to have a power that literally does the EXACT same thing a fighter's power does....and do it even better.