Sean Penn and Bradley Cooper are based off real life celebrities, so of course they would spend more time on them rather than a cameo.
I would argue that the kids do rebel against the restaurant owner by putting flyers in his establishment after he denies them permission. And to a lesser extend that first scene functions as the introduction to Cooper's mother and the kind of circles she works in before going to their meal at the diner where she says she can't travel with him to NYC. I stand by my interpretation when they return to Higgin's character the fact that he has a new wife doubles down on the disingenuous and racist caricature of the man. The absence of subtitles is interesting and I'm curious for anyone that speaks Japanese can translate as we the audience aren't allowed to access their agency here, which is both frustrating and interesting to me.
Regardless, completely respect your indignation towards the two scenes and the character. It's totally problematic, and obviously not fully successful given how it jolts us out of the movie in cheap shock value manner. I just don't think it's as easily binary to chastise but more interpretative than its given credit for on the Internet. Should PTA cut the scene? Of course he can. It's his artistic work. Though on the other hand, he can keep it in and own up to it and all the problematic elements that reside in the movie given its subject matter. At the end of the day, the movie is the movie and what we have is what we can dissect as a work--warts and all.
On a personal note, for sure I was triggered by the accent and it does feel shitty to have these memories of the voice riled up again. I mean, of course it doesn't feel great and it's a place of privilege for one to find the humor here from a disassociated place. It does and should feel bad. Fro me, this is a problematic movie about an uneasy subject matter and the success will of course be subjective. It's a movie that drives on the emotional than a kind of didactic message or theme. For me it's meant to capture a time, a feeling, and a place rather than a traditional narrative. So anyone that feels strongly about it here, completely fair and totally understand your anger and position.
