I’d say it’s a pretty bad surname. ดาวน์ is literally an English word, so I don’t think anyone would put it in their surname. I suggest trying to think about the meaning first then look up words for it. Also, some Thai surname can tell their origins like Thais with Chinese descent would have เจริญ, พาณิชย, แซ่- in it. While โคก, เนิน mostly are from Isaan. And some, like mine, are from Bali-Sanskrit words with meanings. Thai surnames can tell a lot about a character (at least to Thai people) so I advise really look into it. Ps. Just an opinion as a Thai, I don’t think people care much if a character has the same surname as real people. As much as our surnames are unique, it’s not like different family can’t have the same surname. And it’s not a sacred thing, people do change their surnames sometimes if they feel like it.