Yes, creating your own tilesets is easy. More complex tilesets (like the ones shown) can be tedious to create though, because of the shear number of unique meshes they use.
The rule system uses a logic-based approach with various actions that can be performed, e.g. “Set Mesh”, “Set Collider”. MAPGrid also contains documentation that explains the rule system further.
I see, I guess it's similar as with 2D tilemaps, good tilesets require a lot of work but then it's worth the effort.
On another note, for reasons I don't remember, I sent you a email to team@mightyandpowerful.games instead of commenting that here. I could comment here but to not confuse you, I prefer to notify of that mail, it would be weird if I copy and paste it here and then you find the email later
