the mixer is not adding any effects to what you are hearing, the fact that you have a stereo 3.5mm plug connected to the device playing the music is fine but....
the other end is a "stereo" plug which is really called a TRS plug (tip, ring, sleeve) and that cable is reversing the phase of the two channels of music and combining them into a single input, therefore, eliminating all the center channel sounds like: kick-drum, bass, vocals, and anything else that is typically panned to the center when the music you are listening to was mixed.
Using a cable wired like that can definitely fry the device's output amplifier, in other words, the output of the iPad, iPhone, iPod or Android phone can be ruined for life.
The correct way to connect a device that has a 3.5mm stereo output, is with a cable that has one 3.5mm male plug on one end, and two 1/4 inch (two conductor) (mono) male plugs on the other.
You should NOT use a Y cord to combine them together for one input. You need two separate inputs to do this correctly.
And, if you need to play music into one of the XLR inputs, you must have a direct box, preferably two.
Here is the cable you need,
https://www.ebay.com/itm/Hosa-3-ft-3-5mm-TRS-Stereo-to-Dual-1-4-TS-Mono-Breakout-Cable-Tip-Ring-Color/222173063126?hash=item33ba8be7d6:g:aKsAAOSwq8FbL5RB:sc:USPSFirstClass!63031!US!-1:rk:3:pf:0