The other reviews I've read are too harsh. There are far worse movies rated way higher.
This movie succeeds in being light, funny, and entertaining with a dash of mystery. If you were expecting a gritty film noir or a complex mystery from a B-movie titled "Detective Kitty O'Day" then you watched the wrong movie. That's on you.
It's purposefully slapstick and goofy. The cops are completely useless. The boyfriend keeps threatening to walk out until Kitty ropes him back into her antics again. It's great.
I can't think of many other films of the era with a female detective headliner, so in that way it is ahead of its time. The film also reflects the sexism of the time, as they all treat her as hysterical, which makes sense as it was made in the 1940s. That said, there is a nice moment where Kitty's boyfriend sticks up for her.
It was a fun hour spent for me and I didn't feel like my time was wasted.