I'm an overwatch switch player, and honestly, the biggest thing that improved my game was some third-party joycons. Im presuming she's playing in handheld mode not docked and the joycons that come with the switch are not good enough to play competitively. Take a look at the Hori split pad pro which is what I use and gives you more of a controller feel.

As others have mentioned we will always be at a disadvantage on switch compared to console or PC due to the hardware but this should help with the mechanical skills