Corvettes are OP. They absolutely rape Long Range Frigates.
BUT they also are very effective against light frigates since a skilled player will just position his corvette swarm behind the light frigates. Light frigates fire at the front and have a slow turn rate so you can imagine how the fight would go.
Flak is a hard counter to corvettes but since corvettes are faster they can just ignore the flak frigates and go do damage somewhere else. And even when diving in a fleet with lots of flak, it takes way too long for the flak to kill the vettes anyway
Good counters to corvettes are: lvl 6 Marza, Corsev (even at low levels), and kol's.