From my understanding, one can use
StickMount to mount USB Sticks / Thumbdrive. However,
StickMount require one to root phone.
So, are there ways that I can mount a USB Sticks / Thumbdrive to a Android phone / tablet without rooting the phone?
This depends on your device. Sone Android devices have USB ports that can work in host mode or support USB OTG. These devices can at least have read access to the USB drives, some can even use them as full mounts. One app that allows read access on a lot of devices is Nexus Media Importer. You can use it to browse media and copy files to internal storage.