2d6 isn't better than 1d12, its actually worse in terms of statistics. Whiles its max number remains the same, its an increased chance of rolling lower since there are two dice instead of one.

While it is true that the max isn't changed. The min is no longer possible to get a 1. So if you wanted the best damage score it should have been 3d4 

Maybe change the extra damage from one dice size increase to an additional dice, but there may have to be some change to it so that youre not rolling ridiculous damage at low levels with cantrips. I like the idea of this a lot. Here's my suggestion: instead of increasing dice size, add a d4 to all damage you deal with acid damage (1d4=2d4, 1d6=1d6+1d4 etc.) and then at later levels enhance it. So it would be: 

Add an extra d4 acid damage whenever you deal acid damage to a target at 4th level. This increases to a d6 at 8th, a d8 at 12th, a d10 at 16th and a d12 at 19th. Whenever you gain this feat at a higher level, use the dice appropriate to your level.

Something to that effect may get a better reaction but you still keep the damage output balanced and not overpowered nor underpowered.

It would be better to increase the damage by a set amount, since multiple dice can get screwy with balance.