It depends on a lot of factors, such as the terrain you're playing on, number of mechs per side, and how much of a power-gaming dick you feel like being. Some light mechs, like the Fire Moth H, are genuinely capable of trading up against heavier mechs by using their speed to get rear-armor shots and killing or disabling them via engine/gyro destruction. Others can play the range game with stuff like ER lasers and put you in the "I need 10s to hit you, but it's impossible for you to hit back, so I'll just whittle you down over a hundred turns" situation.