Is Silver the Hedgehog an unlockable character in Sonic and All Stars Racing Transformed

No he is not.
As of yet, the only playable Sonic characters are:

• Sonic the Hedgehog
• Miles "Tails" Prower
• Knuckles the Echidna
• Dr. Ivo "Eggman" Robotnik
• Amy Rose
• Shadow the Hedgehog
• Metal Sonic (DLC)