Ubuntu is not an embedded system.
That being said, your question leads us to another one, which is:
What is the difference between an embedded system and an operating system ?
7 years ago this question would be easy to answer, but nowadays the line which separates these two concepts is disappearing behind a haze... By definition, an embedded system is a small, digital electronic device, based in microcontrollers (or DSPs) that perform specific tasks. On the other side, GNU/Linux (and its distributions) are general purpose operating systems used to do important things like access facebook and your email.
But we are not living in 2005, and based in Moore's law I can say that electronic devices are more powerful and smaller than before. This tendency allowed the electronic companies to install Linux in your TV and car, and in a few years(maybe months) you will have Linux in your fridge, oven and underwear (see internet of things). So, now we have Linux installed in small digital devices doing specific tasks...
Beside my fear of being controlled by machines in a near future, I believe the embedded system definition is getting deprecated. So now we need an international organization like IEEE or IEC to redefine or revoke the embedded system definition. Until that happens you can say that Linux(Ubuntu) can be used in embedded systems applications but usually Linux and its distributions are general purpose operating systems.