I've seen a db mock work when 1) there was a small subteam in charge of the OR-mapping, schema structure, and included a DBA; and 2) also a design policy from the architect that all objects had to come out of factories. Under those specific circumstances, having the mock - used solely as a per-developer object cache imitating the factory interface - was critical for unblocking the people working on business logic and front-end.
I wouldn't structure a team that way now.