There are many mods already, but my main question is, are they client or server-side mods? And if they server-side, are they compatible with umod server?
https://valheim.thunderstore.io/

most of them re client side mods.

if you want to use this mods on a server you need to install them on client and on server.

some work without server side mod.

As mentioned above, most of those are client-side and will not benefit from being on the server. uMod is a server-side framework, so anything here would purely be server-side and unrelated to any client mods.

Oh, I see. Thanks for the answer! Does this game do a lot on a client-side? Could be some "Rust quality of life mods" be implemented here on the server-side? 
Auto sort, stack, better loot, auto doors, a full smelter with one click etc.? 

at the moment there is a lot client side. i hope it change in the near future