Honestly speaking? A "flag weight audit" strikes me about as useful to learning from your mistakes as never getting comments on your downvotes would help.
Before I explain my analogy to downvotes, let us first preface this with the issue that flags stick around on deleted posts. For users with less than 10k, they wouldn't be able to see what the flag they made necessarily was, especially if there were multiple problem posts in one question. This makes the audit trail a bit more difficult to follow for some users. There are ways to work around this, but it's still something to be mindful of. Onto the analogy.
Under the current system (as well as how things always were before the flag weight was visible!), you're blind until a moderator contacts you to explain that you've been making a bunch of flagging blunders and should stop. If we had an audit system, then it would tell you the simple essence of "This is what you flagged" and "This is what happened to the flag". It's a bit of light but you still probably need to stumble a lot to get any good learning.
When it comes to dismissed flags, this is the same as getting a downvote with no explanation. A downvoted post tells you in a general sense "Oh, there's probably something wrong in some fashion", but you get no guidance. If you're lucky and insightful, or if the flaw is glaring, you might be able to divine what the downvote was for, and learn. But often a reasonable post gets a downvote and you're simply stumped for what the flaw is after a thorough analysis. It ends up pointless to dwell on it until an explanation is received, and thus nothing is really learned.
Dismissed flags operate the same way. On some clear-cut cases, it will be clear what the dismissal was for. The visible context isn't always enough to explain why a flag gets invalidated, though. Sometimes, you will even get flags dismissed as invalid when you're flagging similar content for similar reasons as your valid flags, because of special circumstances for the particular instance. And there's also the valid point that incorrect flags can still get dismissed as valid because they still led to an entirely different action being taken. Speaking from experience, most invalid dismissals on Gaming that I deal with are less bad flagging practices and more just minor missteps.
The strict values of dismissal don't really give a lot of information as to the reason behind why your flags may have been inappropriate or otherwise improper. If you're interested in learning from your flagging mistakes, the why of it being a mistake is fairly important, and a simple play-by-play audit isn't going to provide that information.