The actress on stage was wearing heavy makeup.

Literal

Stage's top's actress [topic-は] thick makeup [object-を] was-doing.

濃い化粧をしていた — 濃い means 'thick, heavy' (of makeup, color, flavor). 化粧をする is the standard expression for 'to put on/wear makeup.' The relative clause 舞台の上の modifies 女優.