Kaguya is an Otsutsuki, and the Hyuuga are probably a byproduct of deluded genes. *incoming fan fiction theory*
I'm not sure whether the Last talked about Hamura's Otsutsuki branch, but for instance if they came to Earth, it's possible that the Otsutuski whom look like Kaguya, Momoshiki, Hagoromo, etc., all bred with humans which results in the Hyuuga Clan today, which is why they look normal (outside of the Byakugan eye of course) and don't have horns.
Why the Hyuuga Clan isn't as a strong as the Otsutsuki, or the other top tier clans probably has to do with the fact that the Eye itself was never powerful to begin with. It depends on the user. Most Otsutsuki if Kinishiki level, had amazing bodies (strength, speed, and reaction time) or had other Dojutsu (Sharinengan, Rinnegan, etc.,) which coexisted with the Byakugan to enhance their overall abilities. In contrast to the Uchiha, their eyes are powerful to where one can manifest a ability like Kamui, whom was once a weakling before, could now stand against a Hokage level shinobi with the right experience.
Since the Hyuuga Clan has neither (amazing body) or a coexisting power, it's obviously why they are average compared to the Uchiha whom are known for their special eye powers (MS) and the Senju whom have body powers (?).