I think they are making a mistake in saying that Mashima likes to mistreat Lucy and that's why she is not her favorite character, if they see well how the Japanese mangakas are handled, it is very common that their favorite characters treat them quite badly, precisely for not to turn them into Mary Sue, just look at Ken Akamatsu and they will notice.
Lucy sometimes seems to be the one that bad things happen to her, but it is because she is the most identified character with the viewer, is the Normal character in a world of abnormal, and come on, 90% of the readers I I think it would be just as bad if they went to a fantasy world where madness is so common.
Even so Lucy is the most developed character in the series and is fundamental for the development of the same, does not have the super strength of Erza, the huge Hax of Gray, the natural abilities of DS like Wendy or Natsu. She without all of this has been vital to victory on several occasions, without her the Guild as such would have lost to Mard Geer, because it was his invocation that allowed the Celestial Spirit King to free them from the Curse of Mard Geer, without her, they could not have closed the Eclipse Gate and the 10,000 dragons had entered. Without her they would not have been saved from Jacob, or they would have had to face Brandish, without her Natsu would have died when Zeref crossed to him the chest and it would have restarted the time, without her they could not have done the Fairy Sphere and without that act, Acnologia could not have weakened enough that Natsu with the power of the 7 DS could face it.
So really she is very fundamental and maybe as or more important than others, but the detractors go for aspects that are only Plot Comic that does not affect the plot really, but that in the West tend to exaggerate anything that can be used to detriment of a character.
I was planning to put this on before, but the earthquake on September 19th had me out of line.