In my calculations, It's only between 100-150 characters which is very short...

I agree
I want to add song lyrics but there's not enough for all of them

your tagline is just supposed to be a sentence or 2 about yourself or a quote, so there's not really a point

Well, 100-150 characters isn't enough to make sense, Not all users use their tagline for quote or something, It can also be use to promote your stream, You can't just simply convince someone to follow your stream with 100-150 charcters, Anyways I don't see any cons if they do that, Infact making taglines with 200-250 characters can make taglines more interesting...