Don’t think that’s possible to see notes on the detailed view at current. You might want to consider using Device groups to map device(s) to a customer. https://docs.librenms.org/Extensions/Device-Groups/
If your interested in certain interfaces, this is slightly different as they can be aggregated by interface descriptions, e.g “Cust: blahblah” and they will appear under http://librenms/customers
For the additional bookkeeping information, not part of the monitoring software but the feature request you can find here IPAM Integration