Who applied diacritical points in Quaran?

The Qur'an was written without diacritical marks. At the time of Muhammad, Arabic orthography was yet to develop into what we have known for centuries. Al-Hajjaj Ibn Yusuf Al-Thakafi is often cited as the one that applied the diacritical points we see today. He is also credited for introducing the diacritic points to the Arabic script. Al-Hajjaj bin Yousuf applied diacritical points to the Qur'an

For the early interpretors who added diacritical marks, to read the Qur'an as it was originally written, would lead the reader to interpret and choose for themselves from the many possible meanings available.

Muslims began using diacritical marks because reading "errors" began to appear, and the differences this created had led to differences in Islamic law.