There are a few different ways it could work -- one that might be simplest would be to group the scores into 1) mouse only 2) keyboard only and 3) combination. It could be done per song since the information seems to be read currently, or a more ambitious approach might consider them different modes entirely (this seems like a large amount of work, though). And it could be considered for overall pp rankings, the charts every month, or for individual song rankings depending on the preference of the developers and the amount of work involved.
This wouldn't allow a distinction between (tablet + keyboard) and (mouse + keyboard) users, unfortunately, but I'm not sure you could achieve that level of distinction without imposing different modes, and even then I'm not sure whether it's possible or easy to detect whether a tablet is being used vs a mouse.
However, simply distinguishing between the inputs themselves -- namely, mouse only, keyboard only, or a combination -- might be feasible since the data already seems to be there. I'm not sure whether this request entails a lot of work on the development side, but it's something I personally would love so it seemed worth suggesting. It's likely a request that the mouse-only users like myself might appreciate more, as there are very few players (comparatively) that prefer this playstyle (at least at the higher levels, which may or may not include me) and it would be great to be able to narrow the rankings to those on equal footing.