This depends on what type of configuration of SAP Concur you are on. If you are on a Professional configuration you can absolutely build reports in Cognos that show Comments entered on reports by managers.
If you are on a Standard configuration of SAP Concur and you went live prior to November 2017, then you could use Cognos as well. If you are on a Standard configuration and went live after November 2017, then you will not have access to Cognos to reporting.
If you are on a Professional configuration and do not have experience with Cognos, we offer free webinars at least once a month on how to use the tool which provides a good foundation for beginners. You can find the list of offered classes here: https://www.concurtraining.com/pr/reporting/intelligence
PM me if you have any other questions.