On the Network Overview page, connections are identified by IP address, MAC address and hostname. On the List of Configured Clients page, MAC addresses can be associated with a Comment.
My suggestion is to use the Comment associated with a MAC address on the Network Overview page. It could be a new column, but to save on real estate I would suggest using the Comment as a supplement to the Hostname. I would suggest that if a Hostname is defined then that takes precedence. If there is no Hostname then if there is a Comment then the Comment is use in the Hostname column (the column probably wants to be renamed).
I have few Hostnames but plenty of devices with an assigned Comment - it would make it a lot easier to use the Network Overview for analysis if it were not a manual step to compare them.
There is a certain ambiguity rasing from your suggestion.
even when you suggest to do it only by MAC address, Pi-hole supports other client associations, too, like by IP address or entire subnets (e.g., 192.168.0.0/24). The "best" match wins in this case. Looking up by MAC address is tried only when there is no result.
Should we really use the "best matching" comment (when we find one)? In the context of subnets, this may not really be hat you'd expect - but then, maybe it is what a user expect to see, it seems very context-sensitive what to expect where and when.