That's the thing, with characters, stages and DLCs the game's budget is always a good and valid excuse, but not for the lack of a mimic. Mimics are basically almost free when it comes to budget. Every CaS character you can create by default supports every combat style in the game, so turning CaS into a mimic should take like 1 line of code. I bet Inferno model is not much different. There is no budget excuse here, they just arbitrary chosen not to add even a single mimic option at all even though it would have cost them basically nothing to do so.
SC6 doesn't really look like the most content-packed SC game to date to me any more...