I thought my 6th gen battery life was a bit worse than my 5th gen, but it became really bad lately. I use it for about 2 hours a day every day, and it spends the rest of the time in standby (and even when in use, most of the time it's with the screen off just playing music). My 5th gen could last several days like this, with me needing to charge it only once or twice a week. The 6th gen barely lasted more than a day.
I recently did restore of the device and was careful to add back only apps (and settings--like access to notifications and location, etc.) that I needed. I had originally just imported the settings from my 5th gen iPod, which itself may have inherited settings from a previous model. I don't like to do it this way but it tends to work pretty well on iOS (and unlike on a Mac or PC it's more difficult to copy over data after restoring, so it can be more of a pain to do, which is why I don't just start from scratch every time I replace an iOS device). After starting clean, it seems a bit better but I'll see how it fares after a few uses. If you imported a backup from a previous iPod, I might suggest this as well to see if it helps. The only other thing that did for me was turning it completely off when I wasn't using it (in retrospect, Airplane Mode or something may have helped as well, but I never thought of that) since a lot of battery life seemed to have been lost in standby mode. YMMV.