Japanese are serious people which can be a good thing or a bad thing.
Japanese people are known to be serious which can be a positive thing or a negative thing.
この場合、次のような言い方ができますよ。
ーJapanese are serious people which can be a good thing or a bad thing.
「日本人は真面目で、それは良いことでも悪ことでもある」
ーJapanese people are known to be serious which can be a positive thing or a negative thing.
「日本人は真面目として知られていて、良いことでも悪いことでもある」
ご参考まで！
カナダ 
