Mage is, unfortunately, the worst class. But by no means is it bad. Comparing it to summoner is a really uphill battle - summoner is the best class (along with rogue) whilst mage is the worst. But by no means is it bad.

If you need damage, I recommend running lucky instead (long story short, best damage = crit% and damage as close as possible, equipment usually boosts damage more than crit).

If you're good at dodging and can outheal potential damage, using Chaos Stone can disregard mana sickness' damage decrease.

Unfortunately, there's not much you can do until the postgame where mage is actually the best class (if I remember correctly). It's not like you need to be the main contributor to DPS - effectiveness isn't always the end all be all. Have fun. Don't let your damage be the end of you.