While this might be a good idea I do have some stuff to say about it.
You would need to have an application process and requirements, which take time to set up and execute to begin with not even mentioning the amount of time that would go into applications. Starting with major streamers only may help reduce the workload at the start and see if the system works.
Why would the tag be given to people exactly? What benefit will it bring to the community / streamers? There is a live page (https://osu.ppy.sh/community/livestreams) where streamers are shown, what would this tag bring to the table this page doesn't?