Mario Kart Arcade GP
The boss alien from Galaga is featured in Mario Kart Arcade GP as one of Pac-Man and Ms. Pac-Man's exclusive items, unlocked by winning all cups in Grand Prix mode.
Its description in the official game's site is: "Galaga appears. Rival will be abducted".
It also returns in Mario Kart Arcade GP 2 along with all other items, and in Mario Kart Arcade GP DX as a generic item usable by any racer.
Mario Kart Arcade GP 2
The "Galaga Boss" item from the previous game returns, and also Pac-Man's exclusive kart is the Solvalou spacecraft, referencing Xevious, also by Namco.
It also returns in Mario Kart Arcade GP DX.
While published by Nintendo, the arcade Mario Kart games were produced by Namco, so they could freely include elements from Galaxian in the game.