I’m not a pro builder, but hey I think these tips can be helpful even if you don’t consider yourself “good” either.
Be original and unique.
If the theme is “temple”, you don’t have to build a desert temple (most people will). You can be creative, throw in some flavor. This might be hard to do, because it could take time to brainstorm something different despite only having 5 minutes. Some players probably won’t give any credit for your attempt at originality where it’s due (because they just don’t care), but hopefully others will!
Don’t build too big.
If your theme is something like “table”, don’t make the table almost the entire size of the plot. You’ll run out of time, and fail to follow the next tip.
Detail, detail, detail.
Don’t use the just 2 or 3 blocks in your build. If you’re making something wooden, use logs as well as planks, and try different types of wood. Trapdoors, fence posts, glass, banners, etc. are also good for detail. For example, if making a gravel path, a mix of gravel, cobblestone, and andesite gives good variety. Staircases in the ground also give the pathway some definition.
Take advantage of /floor.
Didn’t know you could change the floor from just being white?? Well, now you do. Hold a block you want to use and type /floor. If the theme is something outdoors related, a white floor will be super ugly…
Try building more than just one thing.
If your theme is something like “furnature”, don’t just build a couch and call it done. Add a coffee table, side table, lamp, door, etc.
Don’t stop and think too much. Keep building!
You’re timed. 5 minutes is not as long as you think. Keep building something even if you’ve run out of ideas; it could help improve the detail. Add tall grass, flowers, furnaces, whatever matches the theme.
Have communication during Duos.
If you’re building in duos with another teammate, it’s best that you’re communicating directly such as in a discord, skype, or teamspeak call/channel. Minecraft chat is too slow. Coordinating ideas will get the build done the fastest.
If you think of any other tips I should add, let me know. Good luck improving those builds!