Thanks for your feedback on this suggestion. Bitbucket Server and Data Center 6.9 onwards if committer and author were different, both of their avatars will be displayed on the commit. Hovering on the avatar you will be able to see the committed/authored date.
At the moment Bitbucket Server only shows the author in the UI.
However, Git logs both author and committer. If you make a change and commit, you are both the author and committer. But if a developer cherry-picks a commit, the author is retained and the cherry-picking user will be recorded the committer. (The same is true for rebasing, or amending commits; by default authorship is retained and only the committer is updated.)
Show both the author and the committer in the UI when they are different.
There is no current workaround for seeing this information on Bitbucket Server.