Remember in this case, the mother isn't speculating on what might happen. She might not know everything, but she had an evening with Kaisei and have heard at least some of what happened. She knows he's been bullied and had to move many times, and he made the only connection in life with her daughter only to have to move again. She knows how much her daughter was troubled these 2 years and apparently he too, since he ran away. There's no parent who would see that and say, oh it's just a tail. She doesn't have to imagine what Nachi and her children might have to go through. She's seen it already. She knows it's not what she thinks or what she can come to terms with, which she will because she loves her daughter and will love the ones the daughter loves, but all the others who compose the rest of the world, the ones who have already proven they will react harshly, cruelly, and thoughtlessly.
And because it's an allergory, you're not supposed to fixate on the tail as a medical condition that can be easily corrected (called vestigial tail). It's a placeholder. People have already talked about it being trans, race, physical deformity, etc. What it is, is not important; that it is, is. That's what allegory means. If it is something that would actually interfere with life or is significant in some way, that would detract from the message because it would in some way justify the discrimination by other people. The message is, harmless cosmetic differences should not result in blanket discrimination and harassment. Once you make the difference meaningful (maybe he has really bad BO for example), then you can sympathize with what would be a natural reaction to that difference. So it has to be something innocuous. Again, it's a tail, but it's not. You have to read broadly and see the big picture that was intended.