So, I'm sure that anyone who knows about Fairy Tail has heard this before. And so I'm here to say why this isn't true at all.

Spoiler Warning for the entirety of Fairy Tail and 100 Years Quest.

Now during the first few arcs she is pretty wimpy, but this is explained because as a child Lucy was very closeted and never had to use her magic to survive at a young age unlike Natsu, Gray and Erza.

Lucy is definitely the most powerful Celestial Spirit Mage, as she is able to open 3 Celestial Gates at once, possesses 10 of the 12 Gold Zodiac Keys, and is able to use a fraction of her Spirit's power to use Star Dress which grants her the abilities of her Spirit's.

Now you could make the argument that if you took away Lucy's keys she'd be completely useless, however she's skilled in using a whip. She possesses an extendable magical whip known as the Fleuve d'etolies, with this she was able to defeat Byro who had become a giant octopus monster in the Edolas arc.

Lucy also possesses one of the most powerful magical spells, Urano Metria which she used to defeat Angel of the Oracion Seis.

In the Tenrou Island arc, when taking the S-Class Exam, Lucy had figured out the location of Mavis Vermillion's, the first master of Fairy Tail's, grave with ease through the use of the clues hidden within the Exam.

In the Dragon Festival Arc, she was able to close the Eclipse Gate to prevent anymore dragons coming from the past to the present.

During the Tartaros arc, Lucy had several accomplishments, such as: Outsmarting Franmalth by making him think Natsu was one of Lucy's spirits, becoming the first Celestial Spirit Mage to ever summon the Celestial Spirit King to free her Guild Mates from being trapped in Alegria and defeating Jackal with Urano Metria.

With her newly obtained Star Dress ability which Lucy has training with for a year, she was able to hold her own against members of Avatar and Brandish of the Spriggan 12.

Now for one my personal favourite Lucy moments. She was able to rewrite the book of E.N.D, even though Zeref stated that no one would be able to understand the demonic script and that even if someone did try to rewrite the book, the darkness within would infect and eventually kill whoever tried rewrite the book. Lucy risked her life to save Natsu's but thankfully she had Gray to assist her, and she was able to rewrite the book.

Lucy was also the only person to actually come up with a plan to defeat Acnologia, by having him on a boat so his motion sickness would kick in, and trapping him in the Fairy Sphere so he'd be immobile, and that this would affect his spiritual form, giving the Dragon Slayers a chance to defeat him.

Now in the sequel manga, 100 Years Quest, Lucy can now fuse 2 Star Dresses together to make herself even stronger, so far she's combined her Leo and Virgo dress and her Aquarius and Scorpio dress.