also mole-hill, "mound or ridge of earth thrown up by moles in burrowing," mid-15c., from mole (n.2) + hill (n.). To make a mountain of a molehill "exaggerate an insignificant matter" is from 1560s.
To much amplifying thinges yt. be but small, makyng mountaines of Molehils. [John Foxe, "Acts and Monuments," 1570]