smiling, reed used to cover tatami
莞 centers on a gentle, pleasant expression: a soft smile. The reed sense is a separate, specific plant meaning that does not share an obvious conceptual link with smiling.
莞 combines 艹 (grass/plant) with 完, which likely contributes the sound. The smiling meaning may be a borrowing or extension, but the exact historical development is uncertain.
The grass top 艹 over 完 suggests a plant, but the smiling meaning can be remembered by imagining a face peeking through reeds with a gentle smile.
For カン, picture a smiling person holding a can of reeds: can -> カン, and the smile makes the can feel light.