Margins are low with POD because you aren't doing much of the work, so that makes it all about volume. To get a lot of volume, one will probably need to spend significant time and money on marketing, AND know how to do it effectively. Most people don't understand that last part of the puzzle, including me.
If you have a unique design approach to the niche, you might build a following ... of course, with the POD world being what it is, it will be a following of people selling their version of your ideas, or even just using your art.
It is not, and never was, a get rich quick scheme--despite what some have claimed along the way. I guess they made money selling How To Videos, so they at least got rich ;-)
All that said, lots of people buy pet related stuff. It is a large market. Of course, there are also a lot of people selling into that market. But if all you have is another Pug Life shirt, don't bother. For the price you'll need to charge for a POD produced shirt, you better have a somewhat unique idea/design that isn't already being mass produced and sold for less than you will be paying the POD.