Mii Maker lets you create Miis, like Wii Mii Channel. Here you can transfer Miis over local-WLAN from other systems running Mii Maker(3DS/Wii U), or from Wii Mii Channel. 3DS Miis can't be transfered to Wii Mii Channel, only from Wii.
Wii Mii Channel transfer protocol
The Wii beacons are similar to the usual multi-cart NDS beacons, except: beacon_type is zero, and payload size is 0x14. The payload data is just the Wii UTF-16 nickname, with some extra unused zero data. The usual multi-cast NDS protocol is used for sending the 3DS nick to the Wii. After many keep-alive frames, it eventually sends a bunch of frames, each containing the whole Mii. There's a 6-byte header, followed by Mii data. At the end of these frames like most NDS frames is the 0200 byte marker.
Mii QR Code format
3DS Mii QR is a standard 57x57 pixel Level 10 High ECC QR code with 'Mii' logo in center (refer to Denso-Wave Inc web site for QR Code format specifications). It contains 0x70-bytes of binary data. 3DS seems to have a fully implemented QR-code decoder, as it can interpret such Mii binary data being encoded even in the smallest possible for that data size Level 6 Low ECC QR code.
The data itself is encrypted with AES-CCM, xorpads can be determined from known plaintext here. The Mii Maker application uses the NS APT Wrap/Unwrap commands to encrypt/decrypt this Mii data. For the NS Unwrap command, the Mii Maker application uses blocksize=12, noncesize=10, and inputbuffer-size=0x60. The first 8-bytes(cleartext MiiID+MAC address) in the below Mii data is the nonce used with the NS Unwrap/Wrap commands, the rest of the data at 0x8-0x5F is encrypted.
|0x0||0x4||Mii ID (big-endian 32-bit unsigned integer)
The most significant 3 bits determine whether the Mii is Special, Foreign, or Normal 
time_offset = (mii_id & 0x0FFFFFFF) * 2;
time_offset is the time the Mii was created, represented as the number of seconds since 01/01/2010 00:00:00
|0x4||0x4||High 4 octets of MAC address |
|0x8||0x1||Unknown, the encrypted data begins here.|
|0x18||0x2||Bit-mapped: Birthday (4bit-day,5bit-month), Sex, Shirt, ??|
|0x1A||0x14||UTF-16 Mii Name|
|0x30||0x1||Mii Sharing Value|
|0x40||0x1||Mii Sharing Value|
|0x48||0x14||UTF-16 Author Name|
- QR codes made from the same 3DS for the same Mii are use the same AES-CCM nonce (you can recreate the xorpad by xoring with known values from this table).
The ExtData File System for Mii Maker is as follows:
root ├── icon ├── boss └── user └── ExBanner └── COMMON.bin
|icon||Duplicate from Application ExeFS. Always image 00000002.||0x36c0 Bytes||1.0.0-0||Download|
|COMMON.bin||Extended Banner for Home Menu. Always image 00000003.||0x20224 Bytes||1.0.0-0||Download|