The quality of Peter Gabriel's songs is variable.
There are two periods (IMOO): the inventivity of the first fourth LP's. Then, the rest...
In each subsequent album there are always one or two indigent songs:
I don't know of many artists who don't have one or two fillers on their albums, it's pretty much a given and I don't think it's' reason enough to say that the quality of their work is variable. I also think, in Peter's case that it is quite difficult to lump together the first 4 albums. The first two were clearly the output of an artist who was trying to be original, find his own path while putting the longest distance possible between himself and his previous band. The result was a mixed bag and the quality, as you put it, was certainly not homogenous. PG3&4 were something different altogether, certainly more solid but even on PG3, widely regarded as a masterpiece, you can find at least a song some people consider a filler. SO and US even though very different from another are again of very solid albums material-wise. UP not so much IMO.