You are mentioning ちびまる子ちゃん, right?

If so, I guess "「まる子ウソをつく」" is correct.

Then, it is not "うん" but ";ウソ";. ウソ{Uso} is similar to "ウン{unn}". These are confusing but different.

ウソ{uso} is 嘘, "lie".
So, it is translated as "Maruko lies".

I am very sorry if you were not mentioning ちびまる子ちゃん and "ウソ".

By the way, I have never seen the word "うんをつく", so I believe it must be a typo if "うん" was written in your book.

You are mentioning ちびまる子ちゃん, right?

If so, I guess "「まる子ウソをつく」" is correct.
I guess your book is this. Is this correct?

Then, it is not "うん" but ";うそ";. ウソ{Uso} is similar to "ウン{unn}". These are confusing but different.

ウソ{uso} is 嘘, "lie".
So, it is translated as "Maruko lies".

I am very sorry if you were not mentioning ちびまる子ちゃん and "ウソ".

By the way, I have never seen the word "うんをつく", so I believe it must be a typo if "うん" was written in your book.