No, he was not. I'll give that Sanders did relatively well. However, he did not reach out to the dem's base. It's as simple as that.
No, Sanders didn't do that well. Remember it was a two horse race. His numbers were inflated due to the Caucuses where those who have the most rabid supporters who are willing to drop their lives to go to the Caucus tend to do a lot better than their true support would indicate. When you look at the Primaries which allows everyone to vote by mail or ballot throughout the day rather than having to turn up as a certain time and place, you'll see that his support was not really that high. When it come to closed primaries it was clear to see that he had extremely little support from the party membership.
The only close primary he won in the US was Oregon, and the average points difference for them was 17.2
Sanders won the Caucuses 12 to 5
Clinton won the Primaries 28 to 11
Clinton also convincingly won the Washington Primary despite losing the Caucus there.
Sanders wasn't just down by nearly 350 pledged delegates at the end, but also 3.7 million votes, 12.2% of the total vote!
Compare with 2008 where there was just 62 pledged delegates difference and Obama had just 90,856 more votes than Clinton just 0.26% of the total number of votes.
Had this been a pre-2012 year where California was held on Super Tuesday, the race would have been over in March, but due to it being held so late the Sanders camp just couldn't give up the dream of a big win there that would put them back in the game. It was never going to happen.