I would say at a minimum, Dallas needs to beat any playoff team that has a worse record than they do. That gets them through the WC game at least.
From there, the conversation becomes about the quality of the product on the field. Was the team prepared? Were they competitive? Should they lose in the divisional round, I could see scenarios where he is fired or retained depending on several factors.
IMO— based on talent and performance ceiling, this team should be in the NFCCG. If MM gets them there, then no question he should be retained. Back to back 12-5 seasons is impressive— especially when considering losing the starting LT, QB, and 2nd and 3rd CB to injuries for significant periods of time.