Mages being more level reliant is actually a myth

ADCs, supports and mages on average gain the same amount of DPS per ability rank. Level advantage is not the main reason why mages don't go bot.

The main differences that make ADCs bot lane are:

1. Mages can roam and harass better than ADCs so despite gaining the same amount of damage, they use it more.
2. Mages shove mid lane with waveclear while ADCs need a few items first, this applies even if the mage is at a lower level than the ADC
3. Mid lane encourages trades due to abilities not aggroing minions, the fact wave clear champions tend to have ability harass too, and the fact it's a shorter lane so both sides can be a bit more risky.
4. Mages, from the very beginning (meaning before any level advantage even pops up), are stronger when they're constantly outputting damage until the ADC gets items from first or second back.

This is the main reason why Ziggs doesn't suffer from being underlevelled. Because unless you're like Katarina (daggers gain % ap scaling at level 6, 11 and 16)/Syndra (passive) and actually gain higher AP scaling or power spikes post 6 with your average skill ranking order, you're just as good as an ADC. As long as you're not constantly roaming out of lane or the enemy isn't a solo bot lane mage who's harassing both you and your support out of lane and shoving, a mage will do as well as an ADC.

In fact, some champions like Lucian with his passive are more level reliant than some mages.

Relevant math: Rank 1 to Rank 5 comparisons (ordered from most DPS increase to least) Varus's Q cooldown is reduced to 1/2, and his base damage is 15.66x higher. 31.33x higher DPS Lucian's Q is reduced to 5/9 (above half) cooldown, and his base damage is 2.5x higher, AND his scaling is 1.6x higher. (This one's complicated but I estimate it's around here) base DPS is 4.5x higher MF Q cooldown is reduced to 3/7 (below half) cooldown, and her base damage is 4x higher. 9.33x higher DPS Viktor's Q is cooldown is reduced to 1/2 and his base damage is 3x higher. 6x higher DPS Xerath's Q is reduced to 5/9 (above half) cooldown and his base damage is 3x. higher 5.4x higher DPS Zigg's Q is reduced to 2/3 cooldown and his base damage is 3.4x higher. 5.1x higher DPS Syndra Q has the same cooldown and her base damage is 4.6x higher. Guess her DPS increase.