I'm convinced it's a fetish thing, either on her part, some of her fans or both. I only remember how it started.
>tuned in to lucy, random solo stream, no special event or collab
>suddenly she gets messaged on discord by Bao
>she puts what Bao sent her on-screen, it's the AI baoberry thing, clearly some kind of feeder/inflation fetish animation
>Bao calls her
>the moment people posted this on bao's server, she immediately went around sending this to other vtubers including Lucy
>both spend the next half hour making cheesy fat jokes like "This whole thing has been weighing heavily on my mind." or "I don't think you understand the gravity of the situation."
She's harped back to it and leaned into it so many times since then, with custom toggles or irl cosplay, possibly other stuff, it HAS to be a fetish thing.