Why is benzoic acid a stronger acid than acetic acid? Why is benzoic acid ($\mathrm{p}K_\mathrm{a} = 4.20$) a stronger acid than acetic acid ($\mathrm{p}K_\mathrm{a} = 4.76$), even though the conjugate base in case of benzoic acid is destabilized due to electron donation through resonance. Whereas there is nothing of that sort operating in acetic acid; only inductive and hyperconjugative electron donation.
It is all about the electron withdrawing/ donating nature of the acid's functional groups.  The more electron withdrawing the group, the stronger the acid will be.  The more electron donating the weaker the acid will be.  A phenyl ring is electron withdrawing, while a methyl group is electron donating.  I don't know why resonance would destabilize an organic molecule or ion.  if you are referring to the carboxylate donating electrons to the phenyl ring, I don't see how this could be possible and not violate the octet rule.