Yeah, I don't like this roughy language, it is one of the reason why I dislike reading Maou ni natta Jingai Musume's web novel because they made Yuki talk like a juvenile thug wanna be.
IDK why the author is making the MC struggle with this .. it should literally be his specialty .. is the author stupid?
He's not a designer, his designs came from something that he remembered, not his original creations. Also, his speciality is officeworks not interior designing as shown in his ability to make reports overnight.