Not necessarily. Force is always applied in a direction, and bikes are designed to withstand specific forces in specific directions. Your weight is usually distributed over the saddle and supported by the rails equally over their length. If you hang a 30lb bike on the tip of the saddle, you place undue stress on the rails which transfers to the post.
Truthfully, I'll admit that most bikes won't see damage, but as the kind of person who spends a significant portion of my income on bike parts, I would not treat my bike this way. for 50 bucks, I can get a less attractive mount that holds two bikes without damaging them.
An analogous mistake would be hanging your bike from the wheel. If you have a heavy bike and the wheels are low tension (which would be pretty common for someone who hangs their bike this way, a roadie wouldn't do that), you could stress the wheels and "taco" them (visualize it!) Those wheels are made to handle people over 200 lbs, right? However, they're not meant to support all that weight on a single point - the tension distributes it over the circumference of the wheel.