It is so dull that the mind goes numb.

It means that the tasks described are so boring and repetitious that you fear your brain will go numb from not being used.

Mind-numbingly dull means so dull that you aren't even aware that you are doing it, that you want to do anything else but that. If you listened to a speaker tell in a low about complex national finances for two hours, unless you were interested in such things, it would be mind-numbingly dull. You would be wanting for it to end and be thinking about something totally different from the speech - provided you hadn't fallen asleep already.