Possibly to show the reaction of the actor in the song, which seem to be surprised/heavily pressured by fact of "the demon who wears goddess mask"... maybe... or probably there's no purpose of the kaomoji!.. or it can be anything...
Considering that KOTOKO is a famous denpa song artist (read also What is the actual meaning of "denpa"?), anything goes. (Note: the Japanese title even has a star in it: 七転八起☆至上主義!)
So, what is a denpa song?
Denpa song (電波ソング) is a type of Japanese music that is intentionally strange and catchy. Common features of denpa songs include intentionally off-key vocals, nonsensical lyrics and an over-the-top tune.
Back to kaomoji...
Kaomoji is certainly rare in song lyrics, but there are some more (not necessarily denpa song). From Yahoo! Chiebukuro:
- Koi no Tsubomi: (T_T), (>_<), (*_*), (;_;), (T^T), (T0T)/, (*_*)
- Love Love Chu Chu!: (><), (T_T), (#^_^#), (@_@)
- Happy Succession: (^^;), (T_T)
- girls: (>_<;), (≧O≦)
- message: (-.-;)', !(><)!'
(All links are link to song lyric)
Last but not least, Ryuichi Kawamura has a song titled 〔>_<〕.
Personally, I don't consider Shichitenhakki Shijou Shugi! as denpa song, but perhaps it's because I'm already used to IOSYS.