Recommend Practices and Procedures for Android OS OTA Updates
There is no problem with downloading via mobile data instead of WiFi. The main reason people prefer WiFi is because it is cheaper; WiFi is not generally not metered the way WiFi data is. The update file is downloaded in entirety before the actual update occurs, so the method you use does not affect whether the update succeeds or fails. 

Given the number and severity of vulnerabilities found in software, you generally want to update your Android OS whenever this is possible. Most phone manufacturers do not support phones for long; this period can be only 18 months from the time of release. 

You do not want your device battery to fail while performing an update. I recommend plugging in the charger to perform an update and not starting the update until your battery is sufficiently charged to expect the update can complete on battery alone anyway.