Yes, and no.
Self-motivation, when existent, is more powerful than any external motivational factor, yes. It’s no wonder that “self-motivational abilities” are often mentioned as a characteristic trait/habit of the majority of successful people.
Having said this, I also believe that all humans experience some degree of low motivation at some stages of their lives, as a result of continuous hardships, disappointments or “bad luck”, even when they’re putting in their utmost energy and positive hope in it. In such case, the power of external motivation comes in very handy or even necessary.
Sometimes that inspirational voice in ourselves dies out or is muted down though all kinds of factors and it does no harm in such case to call in the help of external motivators, whether professional or simply existent in our comfort zones; a good friend, a colleague or a family member.
Tony Robbins: Why we do what we do
Motivation has become an industry of itself, with big names like Anthony Robbins inspiring millions around the world with what often sounds like common sense but nonetheless very powerful to hear repetitively. But also through world-famous platforms like TED Talks featuring people from all backgrounds and colours who share the most inspiring words that we need to hear in order to revive our desire to achieve, flourish or believe in ourselves once again.
Dan Pink: The puzzle of motivation