  Differences in the charge radii of mirror nuclei have been recently suggested
to contain information on the slope of the symmetry energy L. To test this
hypothesis, we perform statistical correlation analysis using quantified energy
density functionals that are consistent with our previous knowledge on global
nuclear observables such as binding energies and charge radii. We conclude that
the difference in charge radii between a mirror pair, R_mir, is an inferior
isovector indicator compared to other observables, such at the neutron skin or
electric dipole polarizability. In particular, this quantity correlates poorly
with both the neutron skin and L. We demonstrate that R_mir is influenced by
pairing correlations in the presence of low-lying proton continuum in the
proton-rich mirror-partner nucleus. Considering the large theoretical
uncertainties on R_mir, we conclude that the precise data on mirror charge
radii cannot provide a stringent constraint on L.
