Is CyanogenMod easier to install in rooted device?
Both yes and no. You need custom recovery to flash CyanogenMod. If you have root with the non-recovery method, you can install the ROM manager from the Play Store, which makes the installation of a custom recovery rather simple. 

From there, you just download the CM zip and flash it. Note that the ROM manager does have install ROM as a feature, so you can install from zip and it should automatically reboot to recovery and flash (but I think it works with CWM only, as I have TWRP and it just reboot).