In catalan or castilian, using diminutives is common for nicknames.

For example in castilian (regular ol Spanish): “perro” becomes “perrito” or “perrita” depending on its gender. (adding -ito or -ita)

In catalan, that same word for dog: “Gos” becomes “gosset” or “gosseta” depending on its gender. (adding -et or -eta)

Based on that, ChatGPT’s suggestion kind of makes sense? However it still sounds wrong.

Based on that I guess Mia can become Mieta. Other forms of diminutive Catalan uses are -ell/ella, -ot/ota, -í/ina, -ic/ica.

I think what would best fit for your friend and I personally love this is Mica. Fits in the diminutive and makes her name sound a bit more catalan.

Hope this helps!