Chemistry - Hyperconjugation in cresol
Above are the resonance forms of m-cresol after deprotonation. As you can see, the negative charge resonates around the aryl ring, but never resides on the carbon directly bonded to the methyl group at the meta position.
Thus, the hyperconjugative effect from the meta methyl group does not affect the acidity of cresol in this isomer. Notice how the destabilising hyperconjugation effect would destabilise the resonating negative charge, if the methyl group was at the ortho- or para- position.