Someone who is constantly deceived and never learns would be an easy mark. Here is Merriam-Webster:
: one easily imposed upon, duped, or overcome
The duped meaning especially comes into play in contexts where someone is otherwise being fooled. For instance, Psychology Today in an article titled "How to Keep Fake News Out of Your Head" (Guy P. Harrison, 18 July 2019) recommends that people try to avoid being an easy mark, that is, duped by bad information:
Police your own thoughts. Don’t be such an easy mark. Good thinking is not as difficult as you may imagine.
This word is not only applied to people. A recent headline from CNN demonstrates its use as applied to a country, denoting that a group of followers of the US president thought the country would be easily deceived:
Trump's team thought Ukraine was an easy mark. It was a bad miscalculation. (Nick Paton Walsh, CNN, 21 January 2020)
So Charlie Brown is an easy mark for Lucy's tricks.