If a device is already running cyanogenmod os, is rooting still required? The phone Micromax Yu Yuphoria already comes with Cyanogen Mod OS v12. Does this mean I can uninstall unwanted pre-installed apps or do I have to root the device?
First of all, a correction is required in the question. The device maker Micromax has partnership with Cyanogen Inc. to ship an OS into the former's YU series devices.
What does it mean to me?
Cyanogenmod is a community driven open source Android based ROM. Cyanogen OS (installed in Yureka and Yuphoria) is a proprietary ROM based on Cyanogenmod, owned by Cyanogen Inc., and is used in all those devices that you hear nowadays ships with Cyanogen. (I repeat the stock ROM is not Cyanogenmod but Cyanogen OS.)

Does this mean I can uninstall unwanted pre-installed apps or do I have to root the device?

It means nothing unless one of the partners clear the fog here. Micromax in its Yureka device allowed the users to root it without issues of voiding its warranty, but the device wasn't shipped with an unlocked bootloader, let alone being rooted. There can be multiple reasons why OEMs do not prefer to ship an unlocked/rooted device. Major is the integrity of the phone itself which can be compromised before it reaches user's hands. (Google them)
So, what do I do?
It means you should ask the OEM or Cyanogen Inc. to reveal details, or perhaps, they are already their on web which you may need to search using Google. :)
Ultimately, given the background of the OEM here, do not expect the device's bootloader pre-unlocked or the ROM rooted.