Parents in Sailor Moon is really only seen with Usagi in regards to both her Earth and Moon families, I have nothing against Queen Serenity but her roles are pretty limited to establishing the plot and occasionally messaging her daughter from beyond the grave, that's it; Luna by definition is Usagi's advisor but she should be her real mother.

I have my reasons, the obvious one is well look at her human form, her appearance fits the part of a high ranking figure and wouldn't feel out of place in a crown or royal dress, in fact her dress from the S Movie was made into a real life Wedding dress.
Luna does most of the things that Queen Serenity should do, provide the magical items, advise the scouts and provide them with missions.
She's way more of a motherly figure than even Usagi's Earth mother, it also helps that Luna's original dub voice made her sound like an older lady of nobility.
By extension, Artemis would fit the same role as a father figure but we spend more time with Luna.
Jupiter might be my favourite Sailor Scout but my favourite character has always been Luna.