I admit the rules are not 100% clear on this, but the following one kiiiinda means it’s allowed, I think:
I have code and assets from other Jams I wish to use, are these allowed? Yes! If you have the rights to use it, anything that wasn’t made specifically for this particular jam is allowed. Mini Jam isn’t strict on the “build it from scratch” ruling, but it is encouraged.
We’ve been using premade assets in our previous submission, and have been clear about it (in the Credits section).