Sometimes the file you downloaded is "The.Sims.3.zip" but inside that is another "The.Sims.3.cso". You need to extract the inner file. Use ZArchiver (Android) or 7-Zip (PC). Extract the inner ISO/CSO. PPSSPP prefers standard ISO/CSO over nested archives.
Since this is a fan port, it is often difficult to find a working link. download the sims 3 ppsspp zip file work link
Do not trust YouTube videos promising "Direct Work Links" in the description. These often use link shorteners (adf.ly, linkvertise) that pay the uploader for clicks. Many lead to viruses. If a site asks you to disable your adblocker and then download a "Download Manager" – close the tab immediately. Sometimes the file you downloaded is "The