I assume using that is technically not allowed? It is just a physics engine basically that let's you fool around with digital tabletop components. You can't really do anything there that you couldn't do in the real world.

Feel free to use whatever is comfortable for you to make the game playable. It's not encouraged but you aren't restricted from doing so

I highly recommend though, in the spirit of this jam, that you submit a lo-fi version that allows those without Tabletop Simulator to recreate the experience

This is my own understanding of what is written on the jam page by the way. HeskHwis can probably chime in if I am mistaken :)

I totally agree with evnh! If you feel comfortable with using Tabletop Simulator in your design process, feel free to use it. However, as one of the goals of the jam is to make games that play without a computer, it would be great if you could submit a print & play version of your game to the jam, or detail how to make the game at home for everybody (moreover, Tabletop Simulator is a paid software so as far as I know, people would have to buy it to play your game?).