To lay something on with a trowel is to crudely labour a point, or flatter in an overly generous manner.

From Shakespeare’s As You Like It, 1600.

LE BEAU: Fair princess, you have lost much good sport.
CELIA: Sport! of what colour?
LE BEAU: What colour, madam! how shall I answer you?
ROSALIND: As wit and fortune will.
TOUCHSTONE: Or as the Destinies decree.
CELIA: Well said: that was laid on with a trowel.