no, it just means i put almost as many fights into him past him hitting level 30 as it took to get him to level 30... all the bosses have a max level. i've only hit it on the dark lord and finalmeccia. i haven't needed to fight the other bosses enough since the change to add levels to level the rest up yet. I'd almost like to see them move even girls around more. it shouldn't always have to be dark lord and someone else, make it two, someone others so the rest are getting used equally.
Now, i'm sure the reason for there to always be a girl on the dark lord is for new/low level players. If you want to keep it that way, maybe change him up a bit during events. make him match the level (and coin output) of the max boss the player has access to? Use that as part of why it is harder to get girls out of him as well, but at least then, it is worth fighting him.
*shrug* just my $0.02.