I was thinking of simply going to #Hashtags in the index and being able to select ones to mute/unmute.
I agree with that, but I see it as separate from the question of whether to filter the Messages and Topics lists according to those selections.
The important thing is that online users should have the same functionality as email users.
Currently one only has that functionality in the Topics and Hashtags lists available on the Groups.io home page (click the logo in the upper left). But those lists includes topics from all of your group subscriptions. And because there the Topic mute/follows are separated from the Hashtag mute/follows none of those exactly mirrors what you email inbox would have.