She actually goes "tweet tweet tweet tweet tweet tw-tw-tw tweet tweet" in the lyrics. That's the bridge of "Pack Up".

I presume it is a reference to Twitter, because the lyric as a whole is about not giving a shit about what people say about you on the internet. Or at least I think it is, tbh when I hear the song I am mostly just hearing "bouncy bouncy happy twee yay" in my mind. Except that I like it. In controlled doses.