Should I cover food while reheating it in a microwave?
You do get some reflection of microwaves off a glass or plastic cover (Table 1 here)

    Material    Reflection  Transmission
    Metal       0.88         0.00
    Glass       0.38         0.60
    Plexiglas   0.16         0.83


Don't use metal lids. Neither glass nor Plexiglass plastic absorb much of the beam.

The question then becomes: Does reflection off a lid slow heating more than retaining heat in an enclosed vessel speeds it?
Anecdotal, but experience tells me that using a plate over a bowl or a casserole dish cover routinely gives hotter, more uniformly heated food. Probably true for plastic wrap too, but I don't use that in a microwave since I had a piece of it melt on me.