He found out after marriage that his wife is mentally ill. Should he ask for the mahr to be returned?

No File Exists
Check below answers in case you are looking for other related questions:
Is it permissible to give the mahr in a currency other than that mentioned in the marriage contract?
Paying the deferred portion of the mahr before it is due

No File Exists
Related Answers:
It was written in her marriage contract that she had received her mahr but she has not received it and she has started to resent her husband
In their country the husband is given a dowry!
His father is unable to give his wife the dowry (mahr); can the son give it on behalf of his father?
Is it permissible to give the mahr in a currency other than that mentioned in the marriage contract?
Paying the deferred portion of the mahr before it is due

Recommended answers for you:
The etiquette of eating, with detailed evidence
Description of al-hoor al-‘iyn in the Qur’aan and Sunnah
The evidence that it is obligatory to offer prayers in congregation in the mosque
Qunoot during salaah
Ways of Treating Magic