Do you have extras and extras testing installed?
I recall it being on the software repos.
Follow the instructions in the link below:
https://wiki.maemo.org/N900_The_Perf...l_repositories
Once the repos are installed and the available apps updated you should now see PPSSPP as an option in the software installer app or by installing with apt-get. If you only want stable apps to update and install you might want to disable these unstable and testing repositories.
I am curious if this was worth doing, back in the early days I played a few games with the N64 emulator, native Meamo5 games, as well as a few Palm WebOS hacked games.