Only download PSP homebrew from reputable community hubs like Wololo, Github, or dedicated PSP homebrew archives.
No. Undertale was originally developed using , an engine that does not natively support exporting to the PSP. Official console versions were later released for the PlayStation 4, PlayStation Vita, Nintendo Switch, and Xbox One. Any file claiming to be an "official" PSP ISO is likely a fan project or, in some cases, malicious software. Unofficial Fan Ports and Projects Undertale Psp Iso Download REPACK
Undertale was not officially released for the PSP. Thanks to talented homebrew developers (specifically the port by Vitauct), the game was ported to run on the PSP's hardware. Only download PSP homebrew from reputable community hubs