Yes, you are encouraged to do so. A post-tournament report is great to record key moments in the tournament and congratulate the winners. The report is available to all to read up on.
Go to Manage Events and choose the event in question and select the Report tab. While you can just enter simple text, ManageMyMatch uses a simple formatting language called MarkDown that allows you to better format and organise your text. When you edit your note, a link is given to a web site that will take you through the basics of how MarkDown works.