tea
茗 centers on tea, specifically the young leaves or buds used for high-quality tea. It is a refined term for tea rather than an everyday word.
茗 combines 艹 (grass/plant) with 名, which likely contributes the sound. The character was created to refer specifically to tea leaves or buds.
The grass radical 艹 sits above 名 (name). Picture a famous name-brand tea made from the finest young leaves: 茗 is that premium tea.
For ミョウ, imagine a tea master named Myou who carefully selects the best young leaves: Myou -> ミョウ, and his name is on every tin of this fine tea.