counter for tatami mats, fold
疂 centers on the idea of layering or stacking flat things, like tatami mats. The counter sense is a specific application of that folding or layering concept.
疂 is a simplified variant of 疊, which originally depicted layered fields (田) or stacked objects, conveying folding or piling up. The modern form retains the top 田 and a simplified bottom 冝.
The top 田 looks like a tatami mat, and the bottom 冝 resembles another mat folded or stacked underneath. Picture tatami mats being layered and counted.
For ジョウ, imagine a showroom where tatami mats are displayed in neat folded stacks: show -> ジョウ, and each stack is a counter of mats.