Since version update 2.0.0 added Ranked Match to Mario Golf: Super Rush last month, players looking to attain a rank of A- or higher, were awarded with the means to gain access to different coloured Yoshis.
While the window for obtaining the Yoshis has now expired, now players who achieve the A– or higher grade can obtain a Visor wearing Boo, which can be chosen during character selection.

Source: Mario Golf: Super Rush (Nintendo Switch)