The conjugate base of carboxylic acid is stabilised by two equivalent resonance structures in which negative charge is at the more electronegative O atom.
The conjugate base of phenol, a phenoxide ion, has non-equivalent resonance structures in which the negative charge is at the less electronegative C atom. Moreover, negative charge in carboxylic acid is delocalised over two electronegative O atoms, whereas it is less effectively delocalised over one O atom and less electronagtive C atom in phenoxide ion. Thus, carboxylate ion is more stabilised than phenoxide ion. Hence acids are more acidic than phenols.
