If the design appears obviously incomplete or unfinished, much critical judgement is withheld since it's assumed that whatever criticism they have will be addressed by the final design. Depending on your goal, this can be helpful or even counterproductive (maybe the finished design won't be how they assume it will be). Or others will instead focus on high-level feedback like addressing the overall theme and direction instead of the minutiae of the design.
If the design appears highly polished, then any issue, however minor, is assumed to be "finalized" and thus the criticism pours out.
If you make it appear completed, then people will assume it is in fact completed. If you're still working on it, make it look clearly unfinished.
However, no matter how much you stress it is a prototype or make it look as such, someone will still criticize the the fact it's all in black and white and all the text says "Lorem ipsum".