NOT A TEACHER
I've looked up the word in several dictionaries, including this one http://www.learnersdictionary.com/definition/sucker
It seems that the word might sometimes be informal, but not offensive. Of course, it also depends on how you use the word. If you use it wrong, it may be offensive.
Additionally, you may already be familiar with such sentences as: "It sucks," or "You suck." But according to several dictionaries I've consulted, a "sucker" is not the one who "sucks" in the same sense as in those sentences above. Nevertheless, I think some people might still invent a new meaning for "sucker" as "the one who sucks".
Am I wrong at anything?
Student or Learner