You are confusing a 2-way race with a 3-way race. Mamdani could've won with a lot less than 50% (since there was no ranked choice - and even if it was, I doubt any Sliwa voter would have given Cuomo any 2nd place votes).
The fact that he cleared 50% in a 3-way race is itself a mandate.
The fact that he cleared 50% in a 3-way race is itself a mandate.