Yes, Teresa Teng’s 1980s songs are considered Mandopop. Some of her best songs were originally written for her in Japanese, with her subsequent Mandarin versions released later.
Miki Takashi, the late Japanese songwriter, wrote three consecutive hit songs for Teng. All three were big hits in Japan, which won her the Grand Prix Award there for three consecutive years, an unparalleled feat considering that she was a foreigner in an insular country.
Takashi wrote these songs for her:
1) つぐない / 償還 (1984)
a. In Japanese
b. Mandarin version
2) 愛人 (1985)
a. In Japanese
b. Mandarin version
3) 時の流れに身をま / 我只在乎你 (1986)
a. In Japanese
b. Mandarin version
I also like this song:
海風:
a. (in Mandarin)
b. (in Japanese)
I think the reason why many foreigners consider Mandopop bland is because of the difficulty of understanding the lyrics, which many instances carry subtle symbolic meaning. Chinese lyrics often is poetry. The difficulty of comprehending the lyrics for foreigners may be a factor. Also many Chinese singers do not write their own songs, unlike in the West where many singers write their own songs. Of course Jay Chou is an exception. Why few Mandopop singers write their songs is due to cultural reasons. Asian culture place stronger emphasis on teamwork, collaboration, and reciprocity. Famous songwriters like Luo Dayou and Jonathan Lee write songs tailored to a singer’s specific talent, while the singer delivers for the songwriter.