A master ball is just a regular pokeball with waluigi underwear on it

But Waluigi puts a Г on all of his clothing

No, it’s a W upside down because the Master Ball is held together by Waluigi’s underwear.

Also, if you flip it upside down, it becomes a W, this is a subtle nod to the letter W which is coincidentally part of the english alphabet

here i was thinking it's a majiin buu ball

In addition, the top of the master ball is a subtle nod to someone with no legs wearing panties while being upside down.

In Pokémon, the M on the Master Ball stands for "Mario"