Is there anywhere written in hadith that we have to read Dua Younus for 1,25,000 to get rid of stress or major disaster like Corona Virus? My mother follows a book where it's said to read Dua for specific times but didn't give any reference of Hadith.
The supplication of Yunus (Jonah) عليه السلام is in the Quran (21:87) so it is authentic and supplicating with it is more than permissible as we have been commanded in the Quran to follow the example of the Prophets (e.g. 6:90). Also the use of this dua for alleviation of distress can be inferred from the next verse of the Quran.
فاستجبنا له ونجيناه من الغم وكذلك ننجي المؤمنين
So We responded to him and saved him from the distress. And thus do We save the believers.
وكذلك ننجي المؤمنين أي : إذا كانوا في الشدائد ودعونا منيبين إلينا ، ولا سيما إذا دعوا بهذا الدعاء في حال البلاء
(And thus We do deliver the believers) means, when they are in difficulty and they call upon Us and repent to Us, especially if they call upon Us with these words at the time of distress.
There are also ahadith (see Jami at-Tirmidhi) about using this Dua. I do not know of where the quantity (125,000) comes from but on the face of it there isn't anything objectionable about repeating it any number of times.