This is how kids networks connect with the younger viewers by acting "hip." The sad thing is this was probably made in 2001-2002 since there are 2nd generation of Pokemon on cover and 3rd generation started in 2003 (in America at least) so it is before 2003 otherwise they would have exploited the new Pokemon without a doubt. What bothers me is that there are no mentions of 2nd generation Pokemon in the actual song and Ash's Pokedex had an upgrade at the beginning of Johto Journeys to add new Pokemon thus changing it's voice but it's still Dexter's original voice in the song.
They could have had this on the backburner for years and still thought it was cool to release it in either 2001 or 2002.
But I guess that's when these networks caught up with the 80's and decided "rapping< was the way to connect with kids and that the Pokerap wasn't working regardless of it being lyrically mind blowing.