Would it be bad form to brainstorm before the jam starts?

It is not, and brainstorming before any jam starts is actually a very common strategy! You are free to brainstorm to have an idea for the type of game you would like to make for this event!

Can we make assets before jam starts?

Yes. The only rule is that you cannot do in-engine work until the jam starts! This is due to how RPGmaker devs are expected to be carrying around their own library of assets which would otherwise make a rule against preproduction moot.