Based on the linked (very cool) map, I don’t think what you say is accurate? A flyby of Ceres would be slightly harder than entering Mars orbit, and entering Ceres orbit would be harder than entering Neptune orbit.
Yes for a flyby or orbital mission. I was thinking of a landing mission which costs more delta-V. In practice, you can get that delta-V by aerobraking when going to Mars but not to Ceres, though on the other hand you need to build the spacecraft to withstand the Mars atmosphere. So maybe the answer is "it's complicated" but either way it's reasonably close, not the 5x further you might think from looking at the closest approaches of the orbits of Earth, Mars and Ceres.