Is it ok to sit on the top tube of my Cannondale CAAD12 (2017 model)?
If sitting on your top tube causes damage to your bike, the bike wasn't very safe to begin with. I imagine while sitting you have at least one foot on the ground, so your entire mass is not resting on the tube. 

The amount of force that a frame must withstand is much greater than the weight of the rider. If simply placing 170 lbs as a static load was enough to damage your frame, there would be no way to safely ride the bike.

Update: While not the most scientific test, I decided to try a worst case top tube sitting scenario. I went for a 5 mile ride last night on my alloy road bike. While not as high speed, low drag as the OP's rig, it is all I have. I, a rather large 6'2" 210lbs rider, sat on my top tube while riding for as much of my ride as possible. I do not recommend this, as it turned out to be very painful, and I got some looks form other riders for some reason. Most of the ride was on roads that are not in the best condition (holes, cracks, gravel). Again, I do not recommend this. After completing my 5 miles on the top tube, I brought my frame to work for some x-rays. I x-rayed the entire main triangle, as well as the welds to the rear triangle. I can report that there was no signs of any damage (cracking or deformation of the alloy). There was a bit of damage to me however, as apparently it is a bad idea to sit on the top tube while moving.

Update 2: the mechanical engineers I work with and I conducted an experiment. We got some aluminum alloy tubes (approx 1.25mm wall thickness). It as standard 6061, which is not exactly what is used in the OP's bike. We welded up a main triangle as close to the geometry of the OP's bike as we could. We have learned a few things from this:


None of us should quit our day jobs to build bike frames. Man is that hard.
The strength of the main triangle is insane, assuming the forces are perpendicular to any side. We were about to load a static load of over 350lbs on the "top tube" before it buckled.
Bike frames are not designed to handle rotational forces. The frame will twist up with relatively low force (a torque of about 200lbft.) This force was exerted at what would be the head tube. 
Science is great.


In conclusion, go ahead and sit on your top tube at lights. Since I read this post I have started doing it and it can be quite comfortable.