We are explicitly talking about a mandate - the will of the median voter. Someone who did not receive even half of the popular vote is not representative of the median voter.
I just had to check since I am admittedly sick. Even wikipedia has mandate[1] as
"mandate is a perceived legitimacy to rule through popular support. Mandates are conveyed through elections, in which voters choose political parties and candidates based on their own policy preferences."
Even if we play around with concepts here, in a very, very practical sense, if the mandate is conveyed through elections, at least at the very beginning of the administration, that administration has a mandate to govern. Now.. this perception may change, but you can't honestly tell me this administration has no mandate for one simple reason:
If it does not have a mandate, neither of the previous administrations did.
If he did not secure even half of the popular vote then it is obvious that the median voter does not align with his views.