I think it's fine as long as the license allowed it to do so. Even using Smile Game Builder's Default assets are fine if you decided not to use the "No Default Assets!" theme.
These are the rules related to assets :
"Can I use the Unity DLC to post-process or add resources / assets to my game?" Yes, absolutely! Please do not submit a game ONLY made in Unity, though - make sure you started the project in Smile Game Builder!
This mean that any assets added via Unity are fine as long as the game making process is started using Smile Game Builder.
-No Default Assets! That means no sound effects, models, map graphics, music, anything at all from what comes stock with SGB! This way, we can really show off those DLC packs, other content creators' assets, or make our own!
This is a theme related rule. You can ignore it if you decided not to take the theme. You can check the default assets that are provided by Smile Game Builder here for 3D stuffs and here for BGMs. You can also check here to see how to use your own assets.
"Can I just throw something together?" Boy howdy, CAN you! A lot of game jams (like Ludum Dare) are for people who can throw something together between 48 and 72 hours, and the window for this one is much bigger so that we have plenty of time to take out of our busy schedules. If you need to use default assets, engine-included graphics or music, or decide not to have a battle system just to make sure you have enough time, I'm not judging!
I kinda hard to get what this means. But, from what I get, anything made using Smile Game Builder is fine. This including using the Smile Game Builder's default assets. This also related to this theme's rule :
-These themes suck, I'll just make a game without worrying about them! That's fine! All SGB games are welcome in the 2020 Summer Jam!
I hope that I didn't misinterpret the rules. Hope this helps XD
