I think it is clear that a sucker means somebody who is naive or gullible, while it sucks means “it is bad”, but I wonder where these two terms come from and what they mean originally?
Could sucker actually mean a baby who was just born and therefore, naive and can’t tell if somebody is lying to them? If so, then what about it sucks?
etymonline has for suck:
O.E. sucan, from PIE root sug-/suk- of imitative origin. Meaning “do fellatio” is first recorded 1928. Slang sense of “be contemptible” first attested 1971 (the underlying notion is of fellatio).
and sucker:
“young mammal before it is weaned”, late 14c., agent noun from suck. Slang meaning “person who is easily deceived” is first attested 1836, in American English, on notion of naivete; the verb in this sense is from 1939. But another theory traces the slang meaning to the fish called a sucker (1753), on the notion of being easy to catch in their annual migrations.
Short for "cock sucker", which is the days of non-political correctness and rampant homophobia was a serious and widespread insult/put down. "You suck" having the "cock" understood.
In the pruning of fruit trees and the like, a "sucker" is a branch which forms in one season. They typically fail to produce fruit and so are trimmed off. I would suppose that it's possible this might be how an agricultural people would refer to someone who is too young and not likely to do anything useful.
For example see: http://www.gardeningknowhow.com/trees-shrubs/tree-sucker-removal-and-tree-sucker-control.htm
