This is what I have in CAD currently, and just started printing parts for to test. Any critics or thing I’ve missed that can be improved?
Link to CAD: Onshape
Link to CAD: Onshape
skeletonize the wooden board if you can. or even just 3d print one.
First thing (the big thing) minimize the moment of inertia for those rear wheels. There is probably some optimum for the force of your specific mousetrap, amount of expected friction, size of wheels, etc. This will get you better acceleration. The less mass (especially around the circumference) the better. Shrinking the diameter and using a super lightweight friction material (e.g. latex balloon) may be necessary.
Next thing, loads are small, you likely have more material than you need woth those prints, you can probably get away with very low infill.
It would take some redesign, but a pair of cone bearings (like on a gyroscope) would probably be a lower friction alternative (front and rear axles)
I don’t know if there is another name for these, but it is basically a pointed axle running in a brass cup. The loads are small on a mousetrap car, so you should be able to get away with it pretty easy. You can probably get away with mild steel axle and a brass setscrew with a cup tip
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That’s just the mouse trap
I’ll see if I can actually try to make some of these on the lath/mill this week, never thought of using something like that. Already working on removing more material, and plan on printing wheels in 1.5-3in at 0.25in increments to test.
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oh, well if you have access to a cnc machine, people HAVE actually skeletonized the mousetraps before.
I do happen to have one of those right next to me in my shop… tempting… 
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