I can only speak for the games, since I haven't watched the TV show in so long I can't even remember how long ago it was. But the music in the games started off brilliant early on. The first generation had nice, catchy melodies and were very enjoyable, if a bit simple. The second generation also had great music, perhaps a bit less memorable (but still stuck in my head) but had a bit more depth to them.
After that it all went a bit downhill. The third generation didn't have bad music, none of it was particularly catchy or memorable but it was still enjoyable and set the mood well enough. After that it seems they gave up trying and from then on all the music just seems to me like it had zero effort going in to it. It's like someone just smashed random keys on a keyboard until something vaguely like a melody appeared and then they wrote it down.
I guess it's quite symbolic of the series as a whole. It started off fun and interesting which made me very enthusiastic at the time, but as the series went on there was nothing fresh about it apart from some half-baked gimmick which takes development resources away from actually innovating. Then again I'm not sure Game Freak knows how to innovate anyway.