The answer to this question is yes and no. Linux is the name of an operating system kernel. Linux alone does not make an operating system. Ubuntu is one of the most popular distributors of Linux.
Let us take an example. Let us consider operating systems such as Windows and Mac OS as a mobile phone. Linux is the processor. The processor alone cannot be called as a mobile phone and a processor alone can’t do anything. For making a useful mobile phone you need a battery, screen, a body…etc…A mobile phone is made only when all these components are connected. Ubuntu would be a mobile phone only if it is combined with the processor (Linux) and all other essential components.