I imagine she/he's is a native french speaker. In french a "lambda" object/person is a non-specific, average one.

ah, it's a bad influence from my mother tongue. Lambda means "anybody", but that's just, well, not english :-) I wonder how somebody says it in english.

You might want "layperson", meaning non-expert. Is that want you intended to convey?

I would say the common word is "average".