How could Gen IV be mediocre? It introduced the new Physical/Special split that made Pokemon such as Feraligatr and Absol useful, the first/second best female trainer, an underground mine that makes collecting stones and fossils easier, and a Pokeradar to makes EV training less tedious. Not to mention it helped gave more older gen Pokemon necessary evolutions (Togetic and Tangela were worthless until they've receive major upgrades). The only cons it had was the lack of diversity in electric and fire types, resulting in Volkner and Flint having a horrible team in DP.
I'm not knocking the mechanics. Just the games (D/P/Pt.) themselves. For Pokemon games, I feel they're mediocre. To me, they did not compare to any of the three generations that came before them, or the one that followed.