They just decided to accept that Mapple is broken and to stop caring.
After all, they made a ton of joke skills (like for example, a skill that allows you to eat anything that can fit in your mouth), and Mapple managed to get a lot of those and transform them into game breaking element (like applying the aforementioned skill and applying it to her shield, thus making a great shield that can eat anything that's smaller than it).
And she even managed to break the normal skills.
And the devs can't really nerf those since that would made those skills useless for all the other players.
So instead of nerfing her, they decided to build the game around her.
Like making events where it would be difficult for her to use her skills fully, or make boss that are made to deal with her.