Think of hat or other Add-Ons requiring models or scripts on Blockland servers that are supposed to interact with other players as having a conversation.
Think of this. You are expected to talk to someone in a language they don't know, and they are trying to respond in a language that you don't know. Without a server-based framework and database of hats, your clients would be unable to talk to each other, meaning you don't see the hats. Without a server script linked to a database, the server itself won't even acknowledge that you have hats. The data wouldn't be present on the server, and thus, the server will not acknowledge the command to import hats because there is no mod to authorize it. Take the language brown townogy, and when applying it to a lack of the server side mod, compare it to tying to mail someone a letter by carrier pigeon by forcing it through wires as an electrical signal. You can't send a bird carrying a message electronically through cable. The mediums are not recognized.
It would require a server script to serve as a platform, and an automatically updating database of hats that exist on one server so that everyone can see them, so in other words, it's impossible on the client side. And even when it IS on the server side, it would only be on servers that are running the script and are linked with the database.
To those who script, I hope I got this brown townogy right. Feel free to call me out if it's incorrect.