Seriously WHAT is it? Glad the anime pretty much removed it.
But I still want to know what did it reflect? Why would she want to have her dad as the lover?
As Guardian Princess Nadia saic, it's gross and uncomfortable to witness but writing-wise it fits thematically. Chibi-Usa in the manga feels inadequate in general due to her insecurites over not being able to grow up and the rumors that she might not be the Queen's real daughter. She feels like she'll never measure up to her mom, so what better way to stick it to her than to take her mom's lover? Who also happens to be the 1 person besides Pluto who ever even
tried to understand Chibi-Usa and befriend her. Keep in mind too that as much as it's disgusting either way, in Chibi-Usa's mind she probably makes a clear distinction between Mamoru and her
actual father, King Endymion. Logically she understands they're the same person but emotionally it can be hard to get there, note that she only ever directs her crush at Mamoru, never King Endymion himself. So I think it's safe to say it's not even so much that Chibs has an Electra Complex as it is that she really does feel like Mamoru is his own separate person from her father. To her her father is her father and Mamo-chan is Mamo-chan, a nice older guy who treats her so kindly who she's developed a bit of a crush on. Keep in mind too that Wiseman could've easily perverted her feelings towards him as well and turned what was once an innocent, childhood crush into a warped, twisted thing.
That being said, I always felt a better version of this plot-point would've been Black Lady trying to possess Pluto instead, since remember what triggered her heel-face turn to begin with was the notion that she wasn't the most important person in Pluto's life after seeing her smile and blush at the King. Would've still been gross due to the pedophilia aspect but at least it'd get rid of the incest aspect and would just make more sense in general.