Rainbow Dash didn't get many chances to show off in the first Equestria Girls, so...
Agreed. I remember someone on here mentioned this, but all of the instruments kind of complimented the personalities.
The electric guitar is the "cool" instrument. Rainbow Dash is cool. ( Or she thinks so anyways... )
The drums are considered an energetic instrument. Pinkie Pie is energetic.
The bass is considered a strong instrument, and is also the instrument kind of in the back of music, but always there. Strong like Applejack, and always has your back like Applejack.
Rarity played the piano originally, but switched to the keytar. The piano is a beautiful instrument ( arguably the keytar is too since it's basically the same thing. ), and Rarity is beautiful.
Lastly, the tambourine is a lame instrument. Fluttershy is lame.