Absolutely true, enthalpy. Dipole moment is basically related to charge differential divided by distance. So, the bond length can't be ignored. However, electronegativity of atoms changes to much greater extent than bond length does, so all things being equal the charge differences usually play a bigger role in determining polarity. Usually you will get the right answer in general chemistry if you focus on electronegativity... unless the two EN values are very close together.
Considering that the halogen-carbon bond lengths in bromomethane and chloromethane are about 1.94 and 1.79 Angstrom, respectively, that's only a bond length difference of ~7-8%. By the same token the respective ENs of bromine and chlorine are 3.16 and 3.98. With the EN of carbon being 2.55, the ΔEN for C-Br and C-Cl are 0.61 and 1.43, a considerable difference. For these two molecules, the electronegativity differences of Br and Cl are far more important than the longer bond length imposed by the bigger bromine atom, which is why chloromethane is the more polar molecule despite having the shorter bonds.
These arguments are only qualitative of course and the dipole moment is measured for the entire molecule, not just the carbon-halogen atom. But this is general chemistry so we need not be so focused on fine details.