Is Better Spawner Control a server only mod, or is it needed on client as well?

Since it modifies spawner's behavior, you can assume that its most likely server-side. Also, its mentioned on the mod's page:

Can't always go by that label, I've seen plenty of mods that are required on but client and server with that label. All good tho, I had just tested and tried it out myself. Thanks for the help.