reed used for matting
菰 refers to a specific kind of reed or wild rice plant used for making mats and thatching.
菰 combines 艹 (grass/plant) with 孤, which likely contributes a phonetic element. The plant radical indicates a type of vegetation.
The grass radical 艹 sits above 孤, like a lonely reed standing alone in a marsh. This reed is used for weaving mats.
For コ, imagine a small child (子, こ) weaving a mat from reeds: 子 -> コ, and the child sits among the 菰 plants.