Why?

HCOOH \implies HCOO^-+H⁺...(1)
CH₃COOH \implies CH₃COO^-+H⁺....(2)

The +I effect of CH₃ is more than that of H.
We know that +I effect destabilizes an anion.

Hence CH₃COO^- is less stable than HCOO^- an readily undergoes the backward reaction.

Therefore the forward reaction involved in the dissociation of formic acid is more than that of acetic acid \impliesmore H+ ions are released in the 1st reaction making formic acid more acidic.