Yes, I would say a level 20 fire mage is the worst class in the bracket right now. It's tough because you don't have scorch or inferno blast. Lining up 2 crits in a row with pyroblast would be extremely painful.
I would advise leveling a 29 fire mage. They are extremely mobile because scorch can be cast while moving. Once you do get a crit, you can inferno blast for a quick pyroblast (inferno blast will always be a crit), or stop casting and take 3.5 seconds to pyroblast, then follow it up with an inferno blast. That's 2 major pyroblasts and their dots (for the price of one).
Pyroblast deals about 35% of damage and the dot afterwards is another 20%. Elemental force is up there with ~15% total damage. The dot lasts 18 seconds and can easily be the death of a feral druid or rogue. Though a feral druid with pounce poses a lot of problems for a fire mage. The class mobility makes up for its squishyness, however, I would rank a 100% crit, 29 fire mage as R2 any day.