Recently, in a description of up and down market voice, Miss Snark described herself as 'downmarket'. Several loyal Snarklings rose up in wrath to say Miss Snark was many things but not downmarket.
Let's just say Miss Snark's Easter chapeau had to be enlarged after that round of compliments.
Downmarket is not a comment on quality. Or even a comment on better/worse than upmarket. It's jut a description. Downmarket voice must still be well written. It's not the literary equivalent of "Dogs Playing Poker" paintings sold in the parking lot at the local Piggly Wiggly. (Not that Killer Yapp doesn't pine for one of those of course).
Downmarket is conversational rather than formal. It's not bad versus good. Describing something as downmarket just lets an editor know what style to expect in the writing. It's like telling someone it's fish for dinner not beef so they know to bring white wine not red. (Of course, gin goes with everything.)