Probably, the Mop Fly is not a favored Tenkara Fly Pattern but, perhaps, it should be. Take a look at the water action of the Mop Fly in Fast, Slow and Still Water Action, and you will see it has much to recommend it, and it is about as Simple a fly to tie as there can be.
The Mop Fly came out of International Fly Fishing Competition, where it was so successful there was a movement to ban its use. After watching the video, I believe you will see the reason why that movement came about.
These days, Mop Yarn is being sold through the fly tying materials supply chain. Originally, you had to buy a pair of Mop Slippers from a Dollar Store and snip the Mop Nubbins off to tie the fly with. The Mop Slippers were used to mop floors with and came in only a few very bright colors.
I had never heard of the Mop Fly when I developed my Herl Thing fly pattern, which is very similar in design and concept, but uses only 1 strand of Ostrich Herl, thread, and ribbing wire to tie the whole fly with on a 60 degree Bead Head Jig Hook, and has a much more slender and refined appearance, and also catches fish very well but, is a little more complicated to tie, but not much…Karl.