to mount maps or scrolls, to paste
裱 centers on attaching a backing or surface: mounting maps or scrolls onto a support, or pasting something flat.
裱 combines 衤 (clothing) with 表 (surface, outside), suggesting a cloth or surface used for mounting or pasting. The exact historical development is uncertain, but the component relationship is clear.
The clothing radical 衤 beside 表 (surface) suggests a cloth surface being prepared: imagine pasting a map onto a fabric backing for mounting.
For ヒョウ, picture a hyō (雹, hailstone) hitting a freshly mounted scroll, threatening to damage the pasted surface: hyō -> ヒョウ.