Maybe I'm remembering my PTQ days wrong, but I remember mirror matches coming down to luck, individual tweaks to the playlist, but mostly skill. So if you're achieving better than 50% winrates in the mirror and your deck isn't significantly tweaked to do better in the mirror match, then you're more skilled than most of your opponents.
Hearthstone may be more random than MTG, but in the aggro vs aggro matchup I'd argue that it comes far more down to player skill than anything else. Knowing when to try for board control and when to just race to the face is crucial.