I am leveling an I/L mage and just hit 70. I noticed that even though I have moderate equipment (all together worth somewhat around 2b) I hit very low numbers with the lightning skill and the ice skill. The top I hit with the 2nd job single mob ice skill is around 3000, maybe 3100. With the 2nd job lightning skill the max goes maybe to 2000. This is on mixed golems which are below my level and it feels very low compared to all my other characters that I have somewhat around that level of mesos investment on their gear. When I used the +220 magic attack buff for 2 min I got from the event, I expected some crazy stuff to happen but it was nowhere even near the effect the +100 weapon attack from onyx apple gave to my sniper, and this is 220 not 100 !! I got to around only +1000 +/- increase of dmg with the 2nd job skills which is a joke. I get it that the multipliers are different, but this is so extreme that on level 70 with 2b worth of equips I do max 3100 with single target skill and max 2000 with aoe. It feels like I am missing something.
Mages actually do really decent damage on the early game. What did you compare them to? I think that's decent damage for level 70. Also you can't look at damage linea without considering attack speed. Also, mages are elemental, they each have their elements, take advantage of that and go attack things that are best fit for you
Attackers use STATS*ATK. Mages use Magic, INT+Matk (aka TMA) and INT. A level 70 xbow using a 100 atk Maple Ninshada will double their ATK when using a Onyx apple and as a result deal twice as much damage A level 70 Mage with some decent gear might have close to 500 total INT and a 100 Matk Maple Staff. Using Cheese for +220 Matk increases their range by ~70% While this might feel like a scam (as a mage main, I can confirm that playing one is a scam). Attackers scale linearly with WATK, while Mages scale exponential with TMA. Small increases yield a lot of additional damage some xbow using a 1500 atk Neschere will gain 1600/1500=6.67% dps from using apples a 1500 tma Mage using Apples (+100 matk) will gain ~11.1% dps
One way to increase damage is to invest in the Element Amplification skill. When maxed, you deal 140% damage with all spells permanently while only using 100% MP (early on, the skill will increase how much MP all spells will cost you to use)