Both are important to Linux Mint and get due attention from the developers. Linux Mint developed software like Cinnamon, MDM, Software Manager, Update Manager, and so on, is developed for Linux Mint and both editions benefit from that development. Most programs available in the repositories are maintained by the Ubuntu or the Debian developers, and Linux Mint uses these programs as provided by them. With Debian having over a thousand developers, I'd not call that less supported than Ubuntu
While the main edition of Linux Mint is more popular than LMDE, meaning that the main edition does get some more attention like having more choice when picking an ISO to download, LMDE is important to the Linux Mint team to do right. It has an active userbase, with lots of energetic and knowledgeable folks helping out (some of the regulars you may have encountered already, like zerozero, GeneC, BostongPeng, and Schoelje, to name but a few). UP5 was recently released for LMDE, and UP6 is up and coming later this year and will also see a refresh of the LMDE ISOs.
Though there is no official KDE edition for LMDE, there is an unofficial edition of KDE for LMDE. See Schoelje's work on that here: viewtopic.php?f=61&t=113571
. He's collaborating with the Linux Mint developers and has a while back joined the development team to work on the new Device Driver Manager for Linux Mint (benefiting both editions again).