Nico believes it's pointless to mourn over someone you never knew. He never knew his mother, so no point mourning over her, right? There is no point of missing someone who you haven't met.
And yet, Nico mourns over his mother. He loves his dads, but he wanted to meet her. She was very important to both of his dads, why couldn't he been part of those moments?
She wrote him a letter and left him a handmade blanket.