Hyper conjugation in acetic acid?
Hyperconjugation does play a role in your example.  Acetic acid ($\mathrm{p}K_\mathrm{a}=4.75$) is a weaker acid than formic acid ($\mathrm{p}K_\mathrm{a}=3.75$).  To understand this we must consider the relative stabilities of both the ionized and unionized acids.



In acetic acid there is a partial positive charge on the carbonyl carbon.  Both the inductive effect and resonance effect (e.g. hyperconjugated resonance structures like B) of the methyl group act to stabilize this charge, thereby stabilizing the unionized form of acetic acid.  In formic acid, without a methyl group, similar stabilization of its unionized form is not possible.

Now let's look at the ionized acids.  The negative charge in the anion is delocalized as shown in resonance structure C. This negative charge on the two oxygens makes them much less electron withdrawing.  Consequently, there is much less positive charge on the "carbonyl" carbon in the ionized form.  In turn, there is less need for stabilization of positive charge by the methyl group.  The methyl group does not stabilize the ionized form as much as it stabilizes the unionized form.

Since the methyl group stabilizes the unionized form of acetic acid more than a hydrogen and since the methyl doesn't stabilize the ionized form much more than a hydrogen, there is a higher energy barrier for acetic acid to ionize compared to formic acid.  This means that acetic acid will be a weaker acid than formic acid.