Translation:This is usual of a girl, but not of a policeman!
Imagine that you're a police chief and one of your police officers has been behaving childishly somehow. You might scold them like this.
Or imagine that you see a police officer sitting at the park playing with dolls. You might remark upon it with a sentence like this.
Thanks. I am not a native English speaker, so I would use 'for' instead of 'of'. But I am not sure. Here is a link about this question : https://www.englishforums.com/English/UsualUsualCorrect/mlrgn/post.htm
Yes, I agree with you, "usual for" is better, and is what I would have written myself.
"Usual of" sounds a little foreign, I guess, but it didn't sound wrong enough for me to even really notice it. Just for another example with a similar word: I would also write "typical for..." but "typical of..." doesn't sound really wrong to me, just a little uncommon.
Some English-speakers might find the "of" versions more objectionable and wrong-sounding. I don't know.