Yes, it's weird ! Maybe it's because she is never considered as a Kira by the SPK/Task force ( =/= Kira, Second Kira, X-Kira ) but I personally tends to forget Higushi more, mostly because he doesn't really care about Kira's ideal I guess. To me he isn't really different from the mafia member who wrote in the Death Note. Eh.
Yes, I think you’re right in that it’s partially because she is never awarded a specific ‘Kira title’. Light is the first Kira, Misa is the second Kira, Higuchi is the Yotsuba Kira and Mikami is X-Kira. Though when it comes down to it Mikami and Takada are functionally X-Kira together, because in the end X-Kira is named for the task he fulfills - which is killing while Kira can’t. That Mikami is named X-Kira alone is honestly… nothing but a mistake based on lacking information on the SPK’s part, haha. So for me it’s like… I see why the investigators on the show can’t consider her a Kira until the end, because they don’t know, but for the fandom who does know it’s like… we know she’s a Kira. There is no reason to minimize her role.
For Higuchi, I see where you’re coming from and I myself generally rather group him with the other Yotsuba Eight than ‘Team Kira’ for his lack of personal connection to the ideology (not like Misa is dedicated to it either, but her personal link is there etc.), but he’s still certainly a Kira because he assumed that identity and acted under the name, which the mafia never did.