I don't understand why 10...Qxc6 is an inaccuracy.

Here's the game: lichess.org/KIEynDoV/black#20

The computer says that `10...Qxc6 was an inaccuracy, and that I should have played 10...bxc6 instead. I thought that bxc6 would result in a worse pawn structure (the a6 pawn is isolated), which would become even worse if I play e5 later (the c6 pawn also becomes isolated). Sure, Qxc6 possibly loses a tempo or two against a later Bf3, but it's worth it, right? What did I misjudge?

Center, b-file fun, Qs somewhat exposed (slightly). Mainly the pawn roller in the center.

Three reasons:
1) If you play Qxc6, after Bf3 you have to play Qc7. Bf3 was a useful move, so you actually lost a move in the process.
2) You probably want to play ...d5 in the very near future, so having a c-pawn to help support it is useful. It's also helpful when he pushes e5 to have the c-pawn to create a duo with the d-pawn (if you take with the Queen, the d-pawn can be blockaded easily).
3) The half-open b-file is more useful than the half-open c-file, since a move like b3 is more weaknening than a move like c3.

While you do isolate the a6 pawn, that's a minor consideration especially considering it's rather difficult to put pressure on it, and you can always push it to a5.

Thank you.

Thanks.