I'd think Suisei is the exception here cause she is her own designer, the artists just collaborate with her.
I could be completely wrong and dont feel like going to the wiki to check every single outfit but most Holos have their actual artist make/design their outfits.
Already brought it up once before in here but
this artist worked on multiple suisei outfit, 1 miko outfit, 1 pekora outfit, 1 mel outfit, 1 luna outfit
You activated my autism card:
TAYA (Hinano's mama) worked on Choco's 5th costume
higeneko did one of ayame's outfit, can't find her credentials other than she sell ecchi books
Byulzzi did another ayame's outfit, a lotta yostar/azurlane works
Ceramic did okayu 2nd and 3rd outfit
Kagawa Ichigo did Nepolabo's hoodie and Towa's break your X costume
Hikosan did Pekora's 5th costume and Pekomama's mama
Akai Haato and Haachama have different mama.
Haruyuki designed haato.
sabamisore is credited for "gothic dress costume"
AKIRA EGAWA designed Kobo's 3rd costume. This artist is known for Pokémon TCG Official Illustrators. Worked with type-moon/one piece. Apparently nijisanji managed to score her into doing some Luxiem's voicepack's cover art. Talk about no preferential treatment
Anmi designed Nene's 4th costume, another Arknights mama similar to Sana, she designed
Eyjafjalla
Shiruba designed Kaela's new year outfit. Explaining the omega increase in quality compared to her ugly ass base model
Kawachi Rin designed Bae's 3rd outfit
SWAV did Kiara's 6th outfit, she is also Koko's mama from VWP
Gen 0 wasn't included since they were so old and they all have their base model redesigned by a different mama, including Azki who has like 5, one for every redesigns. Only Roboco from gen 0 didn't change artist.
This is just including credited artists, there are probs uncredited inhouse artist from hololive as well. Like the current case for Kanata.