Here ya go:
#popclip
name: Curly Quotes Gone
icon: symbol:quote.opening
javascript: popclip.pasteText(popclip.input.text.replaceAll(/[“”]/g, '"'))
(The above block is an extension snippet - select it to install the extension with PopClip)
Edit: Here’s how you would generalise it to add other types of quotes:
#popclip
name: Curly Quotes Gone 2
icon: symbol:quote.opening
javascript:
let text = popclip.input.text;
text = text.replaceAll(/[“”]/g, '"');
text = text.replaceAll(/[‘’]/g, '\'');
popclip.pasteText(text);
(note the single quote is escaped with backslash \'
)