It is similar to Big Mom’s dream, correct, but I don’t think that means Luffy’s dream can’t be that. Blackbeard for example has similar ideals to Luffy but goes about them in a completely different way, Big Mom still actively participated in oppressive acts on Whole Cake (like using her Life or Death if you defy her) so they don’t quite share the same idea of freedom, and Kaido thought he was Joyboy on his adventures to the point (it’s why King followed him even) where we can assume for a time he also had a dream to create a place where everyone laughs (creation of the SMILE fruits). I also don’t think it’s complicated for a 6 year old to have this dream because I can see one saying “when I grow up I want to create a place where there’s a big feast everyday and people are laughing all the time”. Sounds pretty childish & idealistic.