In general, I like it when the avatar is used in addition to the username, because I often remember them better.
What I don’t like about the changes is that it is now emphasized several times who did something, but it’s much harder to see what happened. Both the added avatar and the username in bold highlight the user, while the symbol of what happened has become smaller and the title of the topic has lost its color highlighting.
Before, it was “you got a like from username at link”. Now it’s “avatar + username at link, and somewhere in small text, what actually happened.”
Maybe that’s good when you are more interested in “who” did something, but for a general overview of my notifications, “what” is more important for me. When I receive a new notification, the type is more important to me than who; an @mention interests me more than a like, no matter who liked my post.
And the avatars are quite noisy, because they all have different colors. The symbols were less distracting.