From a point of view of formal logic you are right.
Not only from formal logic, but also from historical method. You can't presume to characterize the entirety of the record if you have not seen the entirety of the record.
However if a person could say they have seen a thousand images and they are all or nearly all perfect then they could reasonably say there is something odd going on and requires further investigation.
No. It is not reasonable to let 20% of the data do all the talking unless you can discuss how those 1,000 frames were selected. As you pointed out, this is exactly the problem. The conspiracists do not do original research from the sources. Their exposure to Apollo materials comes from convenience sources and secondary materials -- tertiary in some cases. They look at books published with Apollo photos and presume that those selected photos represent the nature of the entire photographic record. They commit the "loaded dice" fallacy because their argument amounts to, "The few hundred photos commonly selected for publication represent a quality suspiciously high for photos taken in the field." They are suspiciously high quality because that was the criteria by which they were selected for publication from among those many thousands available, some of which are of atrocious quality.
The real question is, "What did you do to assure yourself that you have either seen all the photos, or a sample of them reasonably representative of their overall quality?"
If shown that the archive does contain the usual number of bad shots a died in the wool CT would probably say that they were planted to give the impression of reality.....
And that's fine, because then you have the subversion of support. The conspiracist wants you to explain why all the photographs are suspiciously good. High quality is the anomaly. If the photographs are not universally good, then there is no anomaly to explain. It doesn't matter how the record got to its state of normality. It only matters that it's normal.
Tom: "How did you make your car float six inches off the ground?"
Dick: "It's not."
Tom: "Oh, you probably just weighed it down so that it wouldn't float."
What about a non-levitating car needs explaining?