Ok, this is a simple riddle. As we all know Giorno is a big fan of his “dad” Dio, he is keeping his photo and dyed his hair to looks like padre.They both do “MUDA MUDA” even if they aren’t really related. Mother of Giorno has to told him that dio did scream those words, but she obviously didn’t see Dio in combat. The question is, when Dio has to do “MUDA MUDA” that she saw him. (my shitty english is shitty pls don’t mind)

highdio:

They are really related though,
Dio isn’t a head on top of a mismatched body the way you sometimes
see in fan art, it’s much more complicated since Araki’s version of
vampires replace the body parts of their host with their own cells expressing their own genes.  Giorno’s blond hair is definitely
genetically inherited

from Dio:

image

and it gets expressed around the time his Stand fully awakens. 

Re: your ask, stand cries seem like they can be genetically inherited, it’s why when Jolyne’s stand awakens it shouts ORA ORA ORA even though Jolyne hasn’t seen or heard her dad’s stand yet.  Pretty sure that’s what’s happening with Giorno – Gold Experience instinctively has the same MUDA MUDA battlecry as his dad’s stand.

image

But I’m giving you an in-narrative explanation when it’s also Araki doing a callback.  By giving Giorno so many Dio callbacks he’s emphasizing that this is Dio’s kid.  I think Giorno is a way to explore aspects of Dio’s personality with the evil part written out. 

fwiw, besides saying “MUDA MUDA” Giorno also uses the vampire cry, “WRYYYYYY.”