If Charlotte appears (or is) mercenary, where is the surprise? By and large she has grown up poor (for her station in society), and her plain appearance has let her reach 27 years of age with no desirable marriage prospects. So (presumably after much observation) Charlotte has developed a very pragmatic (if flawed) philosophy of getting a husband and managing a marriage. She clearly does not want to be in Miss Bates's position of being single and poor and likely to grow poorer as she ages.

Thus she engineers a husband and develops a plan not only to manage him as much as possible but to put the family in the way of as much financial growth and security as possible. She does not push Lizzy towards Darcy in place of Col. Fitzwilliam, but she is observant and evaluates the differences. We see from Mr. Bennet what dire prospects may arise from inattention to a family's well being (not to mention Wickham's disregard for his own economic security). So given the choice between neglect and planning, Charlotte plans for the best possible outcomes.