Phenol and phenoxide ion both are stabilized by resonance.
In case of phenol, the lone pair on oxygen atom get delocalize over the benzene ring and this causes a charge separation, i.e., positive charge on oxygen atom while negative charge is on the benzene ring.
This type of resonance structures have small contributions to the hybrid in stabilizing the compound.
While in case of phenoxide ion, the negative charge on the oxygen atom get delocalize and no such charge separation takes place.
Therefore, the resonance structures of phenoxide ion has more contribution towards the hybrid in stabilizing the phenoxide ion.
Hence phenoxide ion is more stable than phenol.