You can incorporate data into many different places on your dashboard: view titles and captions, tooltips, labels (marks and, to a limited degree, reference lines), and dashboard title / text elements (restricted to global elements only, e.g. parameters).
If you're looking to have your comment appear right on the dashboard, you can incorporate it into a view title / caption, or create a view specifically to generate the copy. The latter will consist of a Mark Label consisting of your copy (click the Label box and then click the ellipsis "…" to edit the exact label content), and a placeholder in the Rows and Columns shelves just so you can turn off "Show Headers" to get rid of unwanted window dressing.
DISCLAIMER: The above recommendation is given blind, since you have not included a packaged workbook for review. If you want more specific guidance, you will need to attach a packaged workbook so that we can review your content and validate proposed solutions. For more information: Why we ask for a workbook, and WHY I ASK FOR YOUR TABLEAU VERSION!! For help on creating a packaged workbook, please see the Tableau online help article: Packaged Workbooks
I hope this helps.