It can be done in lower difficulties of course. I did 3 or 4 Mythic dungeons and got 1 fragment each time from the endboss. When we did our Mythic+ dungeons, we got the same amount of fragments as the Mythic+ level we did. So even mythic+2 or +3, which has no affix and is therefore not that much harder than the regular Mythic dungeons, you would get 2-3 fragments each run. In higher level it's just faster.