Month Over Month Usage Report Per CSM

If you have a dataset that has your usage by customer and date, then you can build a report in Report Builder that shows your usage metric (like logins for example) and then is grouped by the date field. You can modify the date field in the Group by to be summarized by month and then change the report to be whatever visual makes sense (e.g. line chart, bar graph, etc.).

As John pointed out, you need the dataset (source) beforehand.
In case you don’t have a source and you need to start capturing the data, I would suggest to take a look at Adoption Explorer to check if that suits your needs.  We have some projects embedded in the C360, are very useful to get insights at Customer Level; and from those same objects you can create reports and dashboards too.  
Hope this helps.