Having the most drafted players and/or undrafted rookies signed by you still in the league means someone, or lots of someones, are doing something right. They deserve the benefit of the doubt on draft day as opposed to meaningless rankings put out by whoever.
I think this is absolutely true, but just playing devil's advocate, I can imagine one way in which it could be a bad thing.
I can imagine a team that makes "safer" picks in the later rounds, guys with high floors but low ceilings, who are more likely to stick in the NFL as backups but less likely to become stars. I don't have any reason to believe the Cowboys tend that way, just noodling.
I do think the Cowboys are an appealing destination for undrafted free agents (a la Romo taking less money to come here over Denver), which is a built-in advantage unrelated to personnel evaluation. But it still means the team is doing something right.