The effects of an aberrated thermal lens on the cavity mode profile and on diffraction losses are investigated numerically for end-pumped solid-state lasers. Planar crystals are considered because they naturally compliment the line-like beam output from diode arrays in addition to offering good thermal management. The aberrations result in a non-Gaussian cavity mode that is significantly broader and thus can cause an increase in the cavity losses. Planar laser geometries can give excellent beam quality. This is a strong function of the degree of the thermal lensing. Thermal lensing must be used to control the overall resonator stability and mode size but diffraction effects, in addition, shape the mode strongly so as to give surprisingly good beam quality for a particular range of thermal lenses.