Maybe dude was swayed by the context, and by the misleading comment that it should be positive, but 'mitfühlend' isn't really the meaning of the word.
A sucker is a gullible or credulous person, someone who falls for every trick, who believes every improbable tale, the ideal victim for a con artist or an unscrupulous salesperson. You can be a sucker for certain things, as in this case worthy causes or touching stories.
Being a sucker is not a positive trait, except in the sense that it's useful to other people who want to get things from the person, such as consent, support, or money. The writer could have used a more positive word, like 'soft touch' or 'soft-hearted,' but he or she didn't. It's possible that in the context of the story the word 'sucker' is used teasingly or affectionately, but the word itself is still negative.
The origin of the slang word is apparently not very clear; the etymology site thinks it could refer to young animals, that is, suckling, or to a fish called a sucker. The actual fish of that name seems to be one that sucks food from the muddy bottom of a pond, but perhaps the figurative image was of any fish so eager to be caught that it attaches itself to the bait like a suction cup, or that takes the bait in a rush, like sucking it down, inhaling it. But that's just a guess.
'Sucker' also means a lollipop (but not anything for babies; that would be a pacifier), but I don't see how that could be connected.
LEO has 'Trottel,' which is not exactly the same but similar, and some translations for phrases in this sense.