This is not entirely correct, since the CD is a single 1.2 mm polycarbonate layer with the reflective layer on top of it, while a DVD consists of the reflective layer sandwiched between two 0.6 mm polycarbonate layers.
But these labels might be causing warping. Many labels will also attract water with time and make balance trouble.
If you remove the labels, you'll see that the quality will most likely go back up and you'll be able to read your data again, maybe not as good as before applying the label though.
Were these by any chance Verbatim Super Azo + Crystal CD-R? The Crystal surface of these is quite prone to those problems.