It's no great work of art, that's for sure. It's pretty goofy. But Monroe does do an amazing job in the song and dance bits. Here is the entire "Diamonds are Girl's Best Friend" number - wow I feel sorry for the women who had to be a living chandelier right in the beginning.
It's really creepy how Monroe's character Lorelei Lee refers to her fiancee as "daddy" - and she does it in other movies too, even doing a song "My Heart Belongs to Daddy" in "Let's Make Love." And she did it in real life too, I believe she called all her husbands daddy at one point or another. I realize it was fairly common during that time period - and it lives on in the expression "Who's your daddy" but it has such unpleasant pedophilic connotations now.
Jane Russell, who was not the blonde, does her own version of "Diamonds Are a Girls Best Friend" plus imitates Monroe's character in a later scene in the movie and she is certainly no slouch in the song and dance department.
According to a Monroe bio, Russell befriended Monroe and tried to get her to join a Hollywood Christians group but Monroe wasn't interested.
The phrase "diamonds are a girl's best friend" acquires a new meaning (and is sung) at the end of the movie when there is a double-wedding for the Monroe and Russell characters.
You can see why Monroe became sick of that character though - she plays virtually the same character in "How to Marry a Millionaire," "The Seven Year Itch" and "Some Like It Hot." And unfortunately for her, they are her best movies - although of course her performance in "Don't Bother to Knock" is her best. And "Niagara" is pretty great too.