I would really hesitate to say that. To my knowledge, there are no
accurate databases to support that statement. Or are there?
Apparently for a long time. See Lehman's Late Maastrichtian
Paleoenvironments and Dinosaur Biogeography in the Western Interior of North
America. Fig 8 indicates a drop in the relative abundance of Triceratops and
an increase in Edmontosaurus from Hell Creek to Lance. Admittedly that's an
old ref but I note Horner's recent work in the Hell Creek yielded
considerably more Triceratops specimens than edmontosaurs.