I suppose one could find Kiyoi's plaid suit with that long shirt "weird," as you said, but it's also high fashion and on point for someone in the entertainment industry. That's why I think the Costume Designer for this show has been excellent. Not just in terms of Kiyoi's high fashion, but even in terms of putting Hira in non-descript, bland clothes, because that matches his character. Hira even says he wants to "be invisible."
As for Koyoma's red plaid jacket, overalls with one strap down, and striped socks, well, he looks like a country boy, and not fashionable at all. But it was also an excellent choice by the Costume Designer because this unfashionable "country boy look" is very non-threatening and approachable. Thus, it fits his character to have created a look for himself that's loud, colorful and friendly. It's a look that says, "Yea, you can talk to me and I'll respond in friendliness."
Kiyoi's high fashion look, on the other hand, says "Don't come near me unless you are on the same level as me." You see, clothes give a message about one's personality, and this is why Costume Designers for shows/movies are so important. Whoever the Costume Designer is for this show is doing a bang-on terrific job.