Steps
After downloading and opening the lyrics.txt I saw somewhat cryptic lyrics of the song:
Rockstar
Pico's a CTFFFFFFF
my mind is waitin
It's waitin
Put my mind of Pico into This
my flag is not found
put This into my flag
put my flag into Pico
shout Pico
shout Pico
shout Pico
My song's something
put Pico into This
Knock This down, down, down
put This into CTF
shout CTF
my lyric is nothing
Put This without my song into my lyric
Knock my lyric down, down, down
shout my lyric
Put my lyric into This
Put my song with This into my lyric
Knock my lyric down
shout my lyric
Build my lyric up, up ,up
shout my lyric
shout Pico
shout It
Pico CTF is fun
security is important
Fun is fun
Put security with fun into Pico CTF
Build Fun up
shout fun times Pico CTF
put fun times Pico CTF into my song
build it up
shout it
shout it
build it up, up
shout it
shout PicoAfter a long while, I decided to check the hint, which suggested mastering rockstar. This little clue helped me to understand that the song may be in fact the esoteric language.
I compiled the song code in Rockstar -> Try It and got this output:
ASCII
114
114
114
111
99
107
110
114
110
48
49
49
51
114
Program completed in 38 msI changed newlines into spaces and put the output into the ASCII to text converter. The flag was mine.
Flag
picoCTF{rrrocknrn0113r}