I also thought about that a long time ago. But IMHO somehow it feels wrong to use component categories to group by manufacturers. Categories and manufacturers are two completely independent properties of a library element and thus should be handled separately I think.
There was already the idea to let library elements have a property for storing its manufacturer. This property would allow LibrePCB to also let you filter library elements by manufacturer. It’s not the same concept as for categories (since it would be based just on comparing strings, e.g. “ST” and “ST Microelectronics” would be considered as two different manufacturers) but I think it would be simple and good enough.