"easily fooled" in Chinese(slang)
很蠢、很笨、笨笨的、呆呆的、没脑子、智商低、白痴、很傻、很呆、脑子不太灵光 etc etc

it depends on the context to decide which one to use...

I don't think 智商低的发指 is commonly used...发指 normally refers to someone/something very bad.