Capable of introducing great games, yes. But I don't think complaints about low quality were claiming the impossibility of great games appearing on GamePass. I think they're rather about the overall quality. And that won't be changed very rapidly by individual good months. For example, if every month repeats the results from this thread--six games with an average score of 84--it would take about two years for GamePass' overall average score to rise to 80.
The current overall average score for GamePass is around 77. Nobody's touting that, because it sounds less impressive than saying "In the last month, the average is 84!" But in fact, I think it's a far more meaningful stat than any short timeslice, which will naturally vary up and down. And an average of 77 is actually quite good for something that's theoretically going to mostly replace buying individual games. Most platform libraries have much lower averages, just because lots of games come out that aren't masterpieces. GamePass next month will almost certainly have a lower average again...but that doesn't matter very much. And neither do above-average release months.
I think this is clearer when you compare how people feel about PlayStation Now. Is their mental image of it formed by the month God of War, GTA 5, Infamous, and Uncharted 4 came to the service? No, of course not. It's formed by the fact that the average overall score for PS4 games on Now is around 68. That's a full 9 points below GamePass, which is a big, meaningful gap. (The lower number is partially driven by a larger library on Now, but that's just a reason the average quality is less, not a rebuttal.)
GamePass' average release quality is more important, and more impressive, than whether a particular short period sees high quality titles.
Note: My stats for GamePass are specifically for Xbox One console games, not including PC-only or prior gens.
