Are you following an account that does good posts, but you don't want to see their boosts of other people's posts?
You can mute just an account's boosts without muting its own posts by doing the following:
1. Go to the profile of the account whose boosts you want to hide
2. Click on the ⋯ button on their profile
3. Select "Hide boosts"
(New people: "Boosts" are the name for shared posts on here. They mean the same as re-tweets or re-blogs.)