You could consider to use the still somewhat experimental alias-clients to group those together.
Note that the DNS protocol itself is void of MAC address information, using the client source IP to identify a requestor, so Pi-hole has to rely on secondary information to associate MACs with IPs.
You should also be aware that MAC addresses may not be as static as you wish for.
Mobile devices like smartphones in particular have pushed randomised MAC addresses as a privacy feature over recent years, and virtualisation software is also known to do so.
In addition, MAC addresses are directly observable on the same link only.
Any L3 switching equipment like additional routers or the fancier varieties of managed switches may split your network into multiple links, as could VLANs. Any cross-link communication relies on IP addresses rather than MAC addresses.
While you would have no problems in observing a unique IP address in such scenarios, even if that's located on another link, the MAC address you see may just belong to your link's L3 switching gateway between those different links, and may thus aggregate a number of different hostnames.