a B grade from Cinemascore is considered to be pretty bad. Only a few movies ever drop into the C range (this includes Zoolander 2, Triple 9 and The Witch in 2016 so far) and is usually a sign that films have been marketed poorly. Having set the wrong expectations for what a film is can yield a very low Cinemascore. For everything else, theres a range from A+ to B-. Batman v Superman getting a B is not a good sign. Its the same grade audiences gave films like Green Lantern and Catwoman. Even Superman Returns earned a B+.
For more recent Cinemascore history, consider The Divergent Series: Allegiant, which opened last week to $29m and earned a B grade. It fell 67% in its second weekend of release, down to $9.5m.