Do you mean as formal dress for shul, a wedding, etc. Very few people consider a straw hat to be anything other than informal wear. In all contexts, wearing a straw hat suggests you consider the activity you are engaging in is not something very important. Straw is a very inexpensive fabric.
Do you mean informal usage? Such as wearing while on a picnic in the mountains?
And how do you define hareidi? If you define hareidi as wearing a black felt hat, then by definition no hareidi wears a straw hat.
If you define hareidi based on hashkafa (being mehudar in mitsvos, and perhaps most importantly, not accept the legitimacy of the Israeli government and holding that in a clash between Israeli law and halacha, we hold by halacha), then many hareidim wear 21st century western style clothes (but they still would not wear a straw hat in a “dress” situation).