Removes uracil from G/U mispairs in ssDNA. Also corrects G/G mispairs. Does not catalyze the removal of thymine from G/T mispairs. Specifically hydrolyzes mismatched double-stranded DNA and polynucleotides, releasing free uracil. Belongs to the uracil-DNA glycosylase (UDG) superfamily. TDG/mug family.