People are speculating why Luffy used G5, some say it's because it's just a normal use of a zoan fruit, some say that it's because he wanted to toy with Lucci, some say that it's because Lucci required G5 to be treaten like this.
Truth is Oda just wants to do this Looney Tunes crap with Luffy
I thought with times it might become a bit more serious but doesn't seem to be the case