You simply could have said "easily taken advantage of", omitting the needless offensive words; you were not in a discussion where it was necessary to provide a root cause diagnosis for why people are easily taken advantage of, nor is your diagnosis especially useful, since the overlap between credulousness and low intelligence is significant but not perfect.

More importantly, in that conversational circumstance, why double down on it? Wasn't your point that we should acknowledge fallibility and use it as a policy premise? Why get off track?