Based on that one Yamamoto doodle comic and how there's only 3 confirmed dexholder Virgos (August 23- September 22), I guess Kusaka likes using the Virgo star sign as a shorthand for naivety (retardation). And it was originally a symbol of feminine naivety in particular.

Sapphire was the first Virgo, her birthday is Sept 21 which is so close to the end of the Virgo time span that makes me sure it's a pointed reference to astrological horoscopes. She's clearly dumb (because childish idealism, though her accent/illiteracy/weird habits don't help) but is otherwise quite independent, especially in the woods. A lot of her story is about her struggle with both independence and femininity. Between herself and Ruby she's somehow the more mentally stable one, she has strong moral values no one can make her abandon.

Whitley is the second Virgo (Sept 16). Unlike Sapphire she seems fine with her femininity, on the surface being a normal kind sweet schoolgirl. But a big part of Whitley's character is her weak minded mental vulnerability, making her dependent on others with better judgement for both physical/mental tasks which is part of why she was a Team Plasma N worshipping cult member. As if she's inherently a "follower" which makes her dumb in a different way. She's almost Sapphire's opposite where Whitley's main strength is her helplessness, her inability to do things/think for herself. Which for a protagonist makes her both interesting and very passive.

So if Sapphire's the retarded girlboss, Whitley's the retarded girlfailure. Two very different ways of depicting a "naive girl" character. It's worth noting there aren't as many female dex holders as the males, but the girls tend to be some of the smartest characters overall, which makes Sapphire/Whitley having "dumb" as character traits look unique in comparison.

Anyway the third Virgo is Sun (Aug 31). He's a guy which broke the girl Virgo trend.... but at least he's also dumb.