The MRCL server is meant to be fully playable just by joining with your Minecraft launcher and accepting the Resource Pack.
However, there are various things you can do in order to improve your experience - both lag wise, and visual wise.
We've uploaded a modpack with allowed and recommended mods on Modrinth: https://modrinth.com/modpack/mrcl. You will need to use the Modrinth launcher to install it.
This includes:
The skies pack is an optional Resource Pack that changes the skybox of Lyoko sectors, in order to create more accurate visuals (as shown in the MRCL trailers).
You need Sodium + FabricSkyboxes to use them. They're included in the modpack linked above. Here's the link: