DTS Neo:6 is a surround mode used to expand stereo sources to the number of speakers configured in a system. The fact that your AVR is applying Neo:6 means your Wii is not sending a 5.1 signal. If you are using analog L/R cables for audio, then stereo is the best you can get. If you are using a digital connection (optical or coax), then set the Wii to output DD 5.1 and play a source with a DD 5.1 soundtrack. Lots of Netflix offerings are just stereo.
Your receiver uses whatever listening mode you have assigned to the type of audio being received. At some point, you probably told it use DTS Neo:6 when the source is stereo. You will need to change the Listening Mode to something else, such as PLII, or to simply play stereo as stereo rather than expanding it to multichannel.