Doesn't really bother me when that happens because 90% of the time I end up with the top Mono score, so I take solace in being better than everyone else using that character at least.
That's assuming the song hasn't been played by 10-12 puzzlers who have all beaten the top Mono score. Which is probably nothing you'll have to worry about unless it's a REALLY popular song. But still. The more time goes by, the more Mono scores will get bumpedd off of the scoreboards.
I don't even play Mono, really, but I feel bad for you guys, and I do think all characters at the same level of difficulty should be reasonably competitive with one another. It may be hard to balance, but maybe Mono's scores should simply be increased (by some multiplier across the board) while the general rules of playing and scoring stay the same?
Failing that, I do really like the idea of being able to drill down to the top scores per character. I know there's a tag where you can play ONLY as mono, but of course hardly anyone else will be using that scoreboard, so it would be really slick if viewing the scores for the song in general, filtered on Mono, would be equivalent to viewing the scores for people who played the song using the everybodymono tag.