***** NOT A TEACHER *****
Hello, BF:
In my opinion, learners should NOT use that word.
As you become more fluent, then you will know when it may be used without being especially offensive or insulting.
Of course, it's always (?) OK to accuse oneself of being a sucker:
Mona: Did you do Pamela's homework for her?
James: Yes.
Mona: Why? You know that she always says bad things about you.
James: I know, but I'm a sucker for blondes. (= When a blonde asks me for a favor, I cannot say no.)
*****
Mona: Do you wear yellow shoes?
James: Of course, not.
Mona: Then why did you buy those yellow shoes today when we went shopping?
James: Because they were on sale at a discount of 50%. You know that I'm a sucker for bargains.
James