You're correct that there is no sufficient reason for anyone other than Minako to have eluded the energy drain in this episode. Good observation!
It is possible that the energy drain only applied to humans, which could exclude the cats. It's possible that the energy drain was localized, whereas only the blackout was citywide. In that case, as long as the girls and Mamoru had not happened to be standing within the range of where the civilians were drained of energy, they could be excluded (along with all other civilians also out of range); it was not specifically stated that the energy drain happened to every single Tokyoite. But they did seem to be in rather close proximity to people who'd collapsed. If the girls had been transformed into senshi form at the time of the drain, we could hypothesize that as senshi they have a higher strength tolerance than regular humans and thereby could retain some energy even if an equal amount had been drained from them, but 1) they were not in transformed form at the time, and 2) even if they had been, in the Crystal canon, Mamoru dresses up as Tuxedo Kamen but he does not transform into it magically as an alter ego in the same way as the senshi do (which he had been able to do in the classic anime series), so Mamoru retaining consciousness would still be unexplained. It cannot be on account of their transformation brooches possessing a special power because Mamoru doesn't own any transformation item.
This does not make sense as a trope of the mahou shoujo genre. Sailor Moon was the first mahou shoujo series which combined sentai (team fighting, such as Power Rangers, usually involving saving the world and protecting civilians from aliens/monsters) with mahou shoujo (which previously had not commonly involved any need to save the world; the classic magical girls used their powers for nichijou [daily life] activities such as disguise, Good Samaritan action, or to carry on a secret career). If other mahou shoujo series that involve team fighting, such as Pretty Cure, jump the shark in the same way as this episode, then it is following suit from the influence of Sailor Moon, which pioneered the combined sentai-with-mahou-shoujo style of mahou shoujo. In other words, this was not a trope in mahou shoujo prior to Sailor Moon, so Sailor Moon needs to have an in-universe explanation for this that cannot be chalked up to a trope.