how to acquire Piko-Piko Hammer/dye in online mod ?

You should use strategy and a little bit of luck is nice, too. Oh, and you must play deathmatches.

Your prize depends on how well you play in deathmatches.
If you place the first during deathmatch, you're rewarded with a randomly chosen joke weapon. One could be the hammer. You can also be rewarded with a rare upgrade, like silk for one of Calista's dresses.

If you place second to fifth, you can be rewarded with a randomly chosen dye. Don't worry, you're not rewarded with the same dye twice.

However, if you place last, you're rewarded with a booby prize - a copper sword.
And just so you know, the prizes also depend on how many are playing along. If it's only you and another player and you place second, you placed the last and you'll receive the copper sword. Better you find a room with more than one player.

Also, in Deathmatches only the looks of armour and weapon are taken over. So you can use a level 1 character, if you wish and still beat someone up who's beaten the game twice.
Boss Battles are a different story. Armour, level and weapon is carried over. But you only receive a weapon for the first win and an item which lets you upgrade said item.