Sona is mostly considered a support because of the buffs she grants from her Aura to nearby allies and with her passive which can be a slow, heal, or damage. Her Q also hits two enemy targets (which can be 2 champs if done properly)
Most support Sona's don't spam her heal too much unless they are behind. She is there to zone the enemy off farm until 6 and provide slight buffs to her ally.
The upcoming changes gear her away from early damage and more towards providing teammates buffs and her abilities scaling a bit better.
I do take Sona mid in normals to screw around, but her wave clear is garbage. Early laning she is fine but by mid game you have no wave clear and its easy to lose towers. Sona with a Meiji's and a maxed E with a alot of ability power is super fun though.