Dress your Palico in either a Mario or Luigi getup, but only for a limited time! This free costume is only available from February 13 to March 15 of 2015. Follow these simple steps to get them:

Make sure you update your game with the 1.1 patch. You can do this by going to the eShop!
Enter your house and talk to the housekeeper.
Select the "Downloadable Content" option.
Enter the Gift Area option on the menu.
Download the free Starter Pack.
Now you have the Starter Pack which consists of the following items:

x50 Mega Potions
x30 Honey
x15 Mega Dash Juice
x30 Well-done Steak
x10 Max Potion
x20 Lifepowder
x5 Ancient Potion
x30 Flash Bomb
x30 Sonic Bomb
x15 Pitfall Trap
x15 Shock Trap
x6 Super Mushroom

The six Super Mushrooms are what you'll need to forge the costumes. Follow the next set of steps to finally be able to dress your Palico as one of your favorite plumbers!

Requirements:

Make sure you have completed the first two quests from the Caraveneer!
Ace Palico must be deployed (change this option on the Palico board in your house).
Have three Large Barrels (you can buy these from the Street Market).
1300z on hand.

The six Super Mushrooms from the Starter Pack.

NOTE: You will only be able to forge one of the Super Mario costumes! You'll either create the Mario costume or the Luigi costume.

Talk to the Smithy (The Man).
Go to the "Forge Palico Gear" option.
For "Weapon," forge the Invincible Hammer.
For "Head," forge either the Mario Cap or the Luigi Cap.
For "Chest," forge either the Mario overalls or the Luigi overalls.
And there you have it! If your gear didn't already equip, you can always return to your house and use the Palico Board to do so.