The CSS provided below will collapse your Mastodon notifications for polls that have ended, favourites and boosts to a single line.
The avatar and action buttons are also removed.
As a result your notificatios are far slimmer, and you can focus on the more interesting notifications, particularly mentions and replies.
You'll need to be Admin of your mastodon instance, though.
To install go to Preferences > Administration > Server settings. Change to the Appearance tab, and paste the CSS into the Custom CSS field.