It's been a while since I've done an LMDE install; however, as I recall the installer is derived from the Debian installer, which has only recently begun to officially support EFI. In pre-EFI versions for at least the last year or two, though, it's been possible to boot and run the Debian installer in EFI mode by setting up an EFI-mode boot loader yourself. The problem is that the resulting install is likely to be buggy; for instance, it wiped the EFI System Partition (ESP) clean, even if it already held another boot loader (such as a Windows boot loader), thus rendering any other OS you might have had installed unbootable. That's very rude. Ubuntu (and presumably non-LMDE versions of Mint) had the same bug until Ubuntu 12.04, when it was finally fixed.
If you've run an installer on an EFI-based machine and it's insisted on creating an MBR partition table, my suspicion is that it was actually running in BIOS mode. That's one of the bugaboos of installing Linux on EFI-based computers -- just because a computer supports EFI doesn't mean it will necessarily boot in EFI mode. Most EFI-based computers also support BIOS-mode (aka CSM or legacy) boots, and if a computer boots in BIOS mode, the installer will probably try to do a BIOS-mode installation, even if that's inappropriate (say, because Windows is already installed in EFI mode).
Linux installers need to get much smarter about these issues, and that is happening -- slowly. In the meantime, the only way for users to cope is to educate themselves. Unfortunately, that's not easy; documentation is scarce and scattered about. My own Managing EFI Boot Loaders for Linux
page is one accessible source of information, but it's more of a technical manual than a how-to guide. Matthew Garrett's blog
is another valuable resource, but as a blog, it's a collection of short essays on random topics, so you'll need to skim the contents and read several entries to begin to learn anything.