At level 100, Magikarp has a total attack of about 22-130. For a lvl 100 Pokemon that is abysmal. In addition, at Gen III, it can learn Splash, flail, and tackle. Assuming that he is using tackle (Unable to read the text) he would have an attack total 165, again, abysmal for a level 100 pokemon.
Not to mention the fact that A: Magikarp at level one hundred would need an approximate 1,500,000 experience points. Even for a simple joke, the standard man would not spend that long to get him to level one hundred. In addition, assuming no EV training has taken place, he would be unable to beat Bruno, as Hitmonlee has a faster speed total than Magikarp. Also, think about it: We never see the stats, we don't see him getting the Magikarp, and half the pokemon it attacked would still be able to fight.
Thus, hakz!