Complex sentence: It was the first film starring Monroe to earn so little money on its initial release, although it was the top-grossing musical of the year and one of only two musicals in the top 20 in 1960.
Split into simpler sentences: It was the first film starring Monroe to earn so little money on its initial release. Although it is important to note it was the top grossing musical of the year; one of only two in the top 20 in 1960.