I don't think Mabel Tanaka should have the robot tag if they don't show any mechanical parts. I've seen 4 post so far where the main character just looks like a regular plain old organic beaver. No screws, no wires, no joints, no sparks, not even glowing eyes. Nothing about these images would indicate that this beaver is a robot in any way.
e621 is pretty clear with it's rule that you should tag what you see and not what you know. Perhaps a robot_(lore) tag would be needed. I tried making one myself, but only admins can make new lore tags.