This is a community event, so all community items should be legal including mods. Using the creative mod does make things a lot easier and quicker to build but it by no means gives you a real advantage over another. Unless you make the whole map covered in gold statues. But that's not very interesting and wouldnt win anyway.
All posted mods are available to every community member, so its up to the player to download and use them or not. I personally don't use mods. I only ever edited the game files once to make burningpet's temple so everything I needed and food were free to make.
Also since this is a community contest using mods will help show it off and may generate more interest in that particular mod and spark ideas for new mods that will further the game's interest. One contest may be exclusively about mods much later on for that purpose. The contest is meant to be for the benefit of everyone.
So to reaffirm, mods are ok, but it would be nice to say what you used for others who may not be aware. So go check out the mods forum if you are interested in using one. The goal of the contest is to make the best and most creative design you can come up with using any tool you have available(without editing the photo outside of the game short of cropping it down to size). I would ask voting community members to not be biased against mod use, sometimes they can be pretty cool.
Just look at the end result and judge it on its idea and execution.
If you are unsure about anything else feel free to post or PM me. I don't want any more confusion and everyone have a good time.
Tomorrow the contest officially starts! Good luck everyone!
EDIT: What Jontis said, the creative mod allows you to explore your creativity without resource limits (in so many words.
). If you don't have much time to play, I encourage you to get the creative mod so you can make your entry in time. If you want to explore your creativity the long way, play the game stock. Feel free to use the whole week to build, or to modify. Just have fun.
EDIT 2: OP edited to reflect saying mods are ok.