can I use assets from lucasfilm games like maniac mansion or zak mckracken?

First, a disclaimer. I'm not a lawyer, and I'm not officially affiliated with this jam (I've been wanting to enter for a while now, and actually might do it this year ... though I seem to say that every year, so no promises).

That having been said, my verdict would be no, you can't use assets from Lucasfilm games. Those are very much the property of Lucasarts, and you do not have official rights to use them. You could potentially make some sort of "fair use" case, especially for a non-commercial, transformative work ... but even that would be very tenuous in this situation, as the jam does have a bit of prize money on the line, and that could put the people running and judging the jam into a bit of a tricky position.

I agree with Aleks. The rules say you can use any tools and assets that you have the right to use. Unless you own LucasArts or can somehow get them to license those assets to you, then you do not have the right to use them, so they are not allowed in this jam.