I'm not sure that's entirely the case, but in terms of user-friendliness, that's certainly true.
I've got Ubuntu 10.10 running on a 1.5 GHz PPC Mac mini. Using Xfce or Openbox, it's snappy and lively for most standard internet tasks. Shortcomings are numerous, though. It's not user friendly.
At all.
Linux on PPC is not for non-hackers. If you don't have a strong Linux background, a lack of good, easy-to-install drivers for the wireless cards in PPC makes it a non-starter for the vast majority of people. Gnash is a poor substitute for Flash. A lack of Adobe Air is frustrating for me since I love DestroyTwitter and TweetDeck. There's still not an actual Firefox 4 port for PPC Linux. Forget about things like good battery management, power stepping for your CPU, or being able to sleep or hibernate.
For me, though, since that mini is a box that's always on and is a CLI-only front for assorted things (it's only running Ubuntu because I love byobu so much and byobu doesn't hook into screen on Macs correctly for some reason), for me it's all a non-issue. If you're talking about a laptop, don't even think about Linux instead of Tiger. You'll probably hate it.