Not sure if this has been mentioned yet, I haven't read EVERY post on this thread, but... I find it interesting that they added NEW mutagens in 1.06, and all of those mutagens are uncommon as well.
I agree that it must be an oversight that they left mutagens uncommon. This essentially restricts their availability to non-mutagenist fields. They dont limit poisons to poisoners, nor elixers of life to cherugeons only. So, it doesn't make sense that only mutagenists have access to mutagens (barring DM intervention of course).
But again, quite odd that they added the handful of low-level mutagens and kept them as uncommon. Hard to determine if the uncommon tag is intended or not.
Curioser and curioser.