Protoss - (Piano): Becoming proficient at Protoss like the piano requires sufficient knowledge of strategy/music and this will make you progress faster in becoming proficient in this race/instrument. Mechanics/physical skill are less important when playing this race/instrument. Piano requires you to play the bass and treble simultaneously, while in Protoss' case it is warping units and producing them out of buildings at the same time. Protoss more than the other two races places extra importance on having the right build/strategy just like a pianist's knowledge of music can make a big difference in the quality of songs that are produced.
Terran - (Guitar): Terran and the guitar are both balanced in terms of knowledge of strategy/music and mechanics/physical skill. One needs to become at least decent at mechanics/skill if they wish to even play the guitar or terran at all, but it isn't an overriding factor. Knowledge of strategy/music does have a moderate level of importance in determining the outcome of battles and the quality of songs. Terran and the guitar are a nice middle ground between the high musical knowledge/strategy requirement to become strong at protoss/piano and the high mechanics/physical skill requirement to become strong at zerg/drums.
Zerg - (Drums): Zerg and the drums require a sufficient level of mechanics/physical skill before you are even capable of playing this race/instrument properly. Strategy/musical knowledge play less importance than the other two races/instruments as being able to produce units effectively and use the drum sticks effectively are of greater importance. Zerg players need to react and follow the opponent's strategy and timings. Drummers like zerg players must also follow the rhythm and pace of the music set by the bassist and the guitarist in the band.
*Note: High level musicians and high league Starcraft 2 players do require high level mechanics/physical skill and high level strategy/musical knowledge regardless of musical instruments and races. In my opinion, even high level musicians and high league SC2 players put greater importance towards mechanics/physical skills or strategy/musical knowledge depending on the musical instrument or race. However, the gap will start to close between the two as the player moves up in league and the musician improves their skill.*