I'll glady share the meaning. It all stemmed from this online 2d over-head game i used to play called "ARC Match", i decided to make a username based off of the TV show "spongebob-square pants". In one episode, a side-kick feels scrutinized by the main hero and decides to turn into a villain. He then proceeds to create his own villan group called "EVIL= Every Villain is Lemons" I thought it was hilarious so i used that very name for the game username. Then it slowly evolved from the entire long thing to just lemons, which then evolved into l3m0n5 (which from afar still looks like "lemons"). And the rest is history.
As for being easy to remember, it actually easier to type l3m0n5 on the keyboard (for me) than lemons. Plus the added use of numbers and letters for wordplay made me look cool.