Green’s Dictionary of Slang

(US) mediocre, second-rate.

1879		Cincinnati Enquirer 7 Sept. 10/7: Tart--Pretty bad, but a little better than snide.
1882		Nat. Police Gaz. (NY) 28 Jan. 3/3: Two young girls [...] are about to become mothers and both lay the blame on a crippled-soldier who has only one arm and a part of a leg [...] Which as a story is very ‘tart,’ young ladies.