Does 12 Angry Men point to the bad jury selection system of the US at the time?

tl;dr: The movie doesn't appear to be an intentional rejection of the US jury system, merely set within the bounds of that system. The behavior of the jury members in the movie is also not indicative of how real juries are instructed to behave, though they are given pretty broad leeway (within limits) to make decisions how they see fit.

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I don't think there was any intention with this movie to call out the jury selection process in the US as "bad". The idea that people should be judged by their peers, and not their "superiors", is a key part of the US legal system that pre-dated the US itself by a very long time. Instead, the movie is an exploration of the idea of consensus building in a place where that consensus has real meaning.

You point out that the jury "almost" convicted an 18 year old of murder; keep in mind, we have no idea if he was guilty or not. The film quite intentionally tells us nothing about the crime beyond what the jury itself knew. They may very well have come close to convicting a murderer of his crime, only to ultimately allow him to go free. That's not a flaw in the jury system, it's a fundamental part of the US justice system: the state needs to prove someone's guilt to the satisfaction of 12 regular, objectively independent citizens before we are willing to punish them for their crime.

As far as the message of the film, it was enough of a validation of the US justice system that it convinced at least one person to become a judge who now sits on our Supreme Court: Justice Sonia M. Sotomayor claims she decided to go into law largely based on Juror 11, who has a monologue in the movie about how much he reveres the US justice system.

It's important to note that many of the things the jury did in that movie are forbidden according to New York City statutes; in particular, one of the jurors did a significant amount of their own research and investigating, which they are expressly warned not to do:

6. Do not attempt to research any fact, issue, or law related to this case, whether by discussion with others, by research in a library or on the internet, or by any other means or source. ^_src

Justice Sotomayor also notes that the behavior of the jury in this movie is something she often warned real juries not to do She does note, however, that as a judge she often instructed juries not to do what the jurors in the movie did, as far too much of their decisions were based on speculation, not factual evidence.