How to make cake mix less sweet
There's not really anything easy you can do. Even adding lemon zest or essence isn't really going to help; it'll just give you a lemony but still sweet cake. I suppose theoretically you could add sour things, since that does tend to balance out sweetness, but it's tricky since acid will interact with the leavening. A sour icing, or a sour sauce to serve it with, might be a better bet in that vein.

So unfortunately, you'd be best off shopping around for cake mixes that are more the level of sweetness you want. You might be able to use the nutrition facts to help get a sense of how sweet they are before you buy them.