Answer
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
In this fatwa:
The name al-Mahi refers to the unique role of the Prophet as an eraser of disbelief; as such, it is too pompous to name your daughter al-Mahi.
Responding to your question, Sheikh Ahmad Kutty, a senior lecturer and an Islamic scholar at the Islamic Institute of Toronto, Ontario, Canada, states:
Children are gifts from Allah
Children are one of the most beautiful gifts of Allah; as such, we ought to thank Allah for it. They are a great trust in our hands, so we are obligated to do everything within our means to ensure that the children are nurtured to become responsible Muslims. This cannot happen unless we take all the necessary steps to set them on the straight path through our actions and practices at home and outside.
Etiquette of naming children in Islam
One of the first duties we owe the children is to give them good names and ensure they are brought up with the best manners.
Prophet Muhammad (peace be upon him) said that the best names are names such as Abd Allah, Abd al-Rahman, etc. (that connect a person with Allah); the most fitting names are Harith and Hamman. You may also name your children after the names of prophets.
Can you name your daughter Al-Mahi?
However, it would not be desirable to choose such names of the Prophet that describe his specific function. For instance, the name al-Mahi refers to the unique role of the Prophet as an eraser of disbelief; as such, it is too pompous to name your daughter al-Mahi. So I would urge you to look for another good name. You should be able to find one.
Almighty Allah knows best.