I’m not a fan. [name_m]Matthew[/name_m] is strictly masculine to me. I’m not opposed to [name_u]Mattie[/name_u] as a full name for a girl though, if someone wanted to honour a [name_m]Matthew[/name_m]. It’s a nice nickname for a [name_f]Matilda[/name_f] too.
I live in the UK, so [name_u]Mattie[/name_u] is pronounced quite differently to [name_f]Maddie[/name_f].