Should you tow? That’s a personal choice. We’re supposed to read the owners manual and follow the recommendations provided by the manufacturer. Knowingly towing a trailer on public roads with a vehicle not rated to tow, may not be the wisest decision one could make.
There are different issues involved here. A tow rating is a least partly a rating of how much weight the rear bumper or rear suspension of the vehicle can safely support. If you're towing a single-axle trailer that's too heavy, it may weigh down the rear axle of the towing vehicle so much that it's not safe to drive it, either because the rear suspension is overloaded or because the front wheels are lifted so much they don't provide proper traction for steering.
Also, since the Clarity PHEV is (at least from what Mr. Google says, so correct me if I'm wrong) a front-wheel drive car. So weighing down the rear axle will also reduce the traction the car gets on the road, making the problem with traction when towing even worse.
For two-axle trailers, there may be a problem of sheer mass. To give a very clear example, a Chihuahua couldn't properly handle a normal dog sled even if it was going downhill. The tiny dog simply doesn't have enough weight/mass to be able to steer the sled, nor enough power to slow the sled if it starts running too fast downhill. Precisely the same thing occurs for a large, heavy trailer towed by a car or small truck, if the trailer is significantly more massive than the tow rating. At highway speed, it may not be possible for a small vehicle to change the angle at which the trailer is traveling, or the vehicle may not be powerful enough to drag the trailer up a steep hill... or even more dangerously, not stop it from accelerating when going down a steep hill.
If the vehicle has no tow rating, then use common sense. A small car can't safely tow a big heavy trailer at highway speed, regardless of any tow rating it may or may not have. Limit the size/weight of the trailer to a reasonably small fraction of the size/weight of the car, and you should be okay. Overloading the rear bumper or rear suspension should be pretty obvious by how much the rear end sags under the weight of the trailer. If the front of the car is obviously being lifted, then you've obviously overloaded the car, even if the wheels are still on the ground.
A relevant quote:
How much your car can tow depends on the type of vehicle and the options on the car or truck. A general rule of thumb for a front wheel drive car or small SUV is 1500 pounds, a mid-size all-wheel drive SUV or pickup 3000 pounds and a full size SUV or half-ton pickup truck 5000 pounds or more.
https://camperreport.com/how-much-can-my-vehicle-tow-complete-guide-to-safe-towing/