。
Last night wasn't too hot.
Literal
Last-night [topic-は] too-much hot was-not-[past-かった].
Simple weather-comment with the あまり~ない pattern. 暑くなかった is the past negative of the i-adjective 暑い ('hot' for weather/ambient temperature) — distinct from 熱い ('hot' for objects and liquids). Remembering the distinction 暑い (weather) vs 熱い (thing) is essential.