The new boss, Coren Direbrew, is pretty cool and I'm happy to see older content being made relevant again (especially when it's an instance as cool as BRD), but as far as I can tell from the forums and various data sites, the drop rate on the mounts is...less than spectacular. I'm seeing estimates that range from as little as 1% to as much as 15%. Blue poster Kisirani pops up to say it's "much, much easier" to get a Brewfest mount drop than it was to get the Sinister Squashling off the Headless Horseman. Wowhead lists the Squashling as a 2% drop, so maybe the 15% figure for the Brewfest mounts is more accurate.
The mount might be a crapshoot, but you can cover the other bases on an upcoming Achievement and title by planning ahead: