I would say that the "wimpiest" would simply be the one with the least real substance; generally it would have a pretentious air of sentimentality, but not a darn bit of actual meaningful content. A song could have a strong, clear meaning, but be crappy because it is very sloppily written, or it has a less than mediocre melody, or it expresses crass jingoistice "patriotism", or for who knows how many reasons.