In case of monopole array antenna, I think would be better to use a common ground plane instead of using separate ground planes for each antenna.
If the ground plane of a monopole antenna is asymmetrical and have some lengths (distances from the antenna) smaller than λ/4, then the pattern may be changed from omnidirectional. If all the asymmetrical lengths of the ground plane are greater λ/4 then the pattern would be close to omnidirectional.
If use a solid common ground plane on the top layer, doesn't matter if use or not a bottom ground plane.