Interestingly, the Chinese glyph (疊) from which this glyph was derived does not refer to tatami or matting at all—the word 'tatami' and its meaning were added by the Japanese many centuries later. This glyph must have meant something else formerly, such as「plugging orifices surrounded by abdomen, meat pile」and after use, FOLDED INTO LAYERS for storage (TO SHUT, PUT AWAY) and REPEATING, DUPLICATING use of the same mat (and so on) again and again.
Moreover, the outside surfaces of tatami mats are not woven from rice straw at all (too brittle, too short) but rather, from split igusa reeds…rice straw may have been bundled and simply packed within the woven reeds, but more likely the filler was some sort of batting or hemp cloth since dry straw would quickly flatten and crumble after only a few uses, losing its effectiveness for repeated use (today, the filler-batting is usually replaced by synthetic materials). Moreover, tatami mats cannot be folded, but some other type of softer mat could be FOLDED IN LAYERS.
Nevertheless, top element is「wetland」where both the reeds and rice straw can be found, a「filled orifice」(in the source glyph, tripled as in very many of them). Note that in Japanese vernacular, the word for「wetlands」manko 漫湖, refers to a vagina as 'the other orifice' for filling and plugging within a wet area:「wetland plugging orifice(s) temporarily surrounding, meat piled」indeed suggests service providers lying upon MATS, MATTING:「abdomen surrounded on all sides with rod array」as depicted in erotic Chinese art, with men in dynastic China masturbating by wrapping no-longer-integrated feet and other of her naked body parts around an erect penis and spurting onto her belly, a fraternal activity still popular today for that matter (without the no-longer-integrated feet, naturally).
Glyph in Japan depicts the mat itself, and not the plant material from which the mats were made, as perhaps another oblique reference to the USE OF MATS upon cavern floors, where sat the shaman's service providers each with her wetland atop a mat,「plugging orifices, surrounding piles of meat」or「filling orifice properly」by using a mat beneath.
Older glyphs all with the same meaning include「many crotches accumulated in meat pile」(叠) and「alternating between many plugged orifices」(疉) that leave little if any doubt as to what the glyph was referring. Also meaning REPEATED USE, REPEATED ACTION, BENT OVER and ACCUMULATED, clearly a description of a bunch of folks in a FOLDED, BENT-OVER, LAYERED position revealing groins for repeated and easy access by a gang of buddies invited over for the festivities and beers, but likely situated on MATS rather than the rocky cavern floor as perhaps the only connection to what later became tatami-like mats.
USING MATS necessary for sexual activity would certainly make everything more pleasant for all involved and reduce the number of skinned knees and elbows. The matting could be easily FOLDED or FURLED and put away when not in use, but tatami mats are unbendable and are not removable when not in use.
Nevertheless, some still claim that the top element as「rice paddy」is an oblique reference to growing rice plants, and growing rice plants as an oblique reference to harvested rice as an oblique reference to the residual straw by-product, a material that was purportedly used for stuffing mats, folding woven single layers of rice straw into thicker layers, and finally surrounding the thickly folded, piled up layers into a mat, none of which is the actual production method of tatami matting.

External source definitions …excluding politically incorrect concepts and other meanings deemed offensive today; may include extended definitions, euphemisms, and mnemonics)
Chinese: Japanese variant of 疊|叠, to fold; to fold over in layers; to furl; to layer; to pile up; to repeat; to duplicate; Japanese tatami matting
Japanese EDRDG: tatami mat, counter for tatami mats, fold, shut up, do away with
Unihan extended: repeat, duplicate; repetitious