The rotation cobbled together at the start of the season lacked a top of the rotation. It was a pair of 4s, a pair of 5s and a 3, and the 3 was a borderline 4. You can't win with that. 4s and 5s are not mediocre. Generally, they suck. When combined with top of the rotation pitchers the overall result usually averages out to mediocre, I.e. League average. A rotation comprised solely of 4s and 5s will usually combine to produce a bad result.
To this point our two 4s, two 5a, and a 3 have performed like two 5s, two DFAs, and a 2/3 (Buch).
Edit: btw, the 1975 Reds had a great top of the rotation pitcher in Don Gullett. His career was cut short by injury, but he was very good -- a star that was still on the rise when injuries ends his career.