Nothing remotely wrong with trying to appeal to a broad spectrum of people. On a practical level, doing so is essential to winning elections. You'll never form a Government if you only appeal to 20% of even 30% of the electorate. But there's also a moral issue: If you want to Govern, you should be aiming to help - as near as possible - all the people. Obviously in practice, helping literally everyone is probably impossible, but as an ideal, it's what you should be aiming to try to get close to if you want to form a Government that is actually going to do good, and not end up dividing the country.
The Conservatives have amply shown us the alternative time and time again: Appeal to just enough people to get elected and then work on the basis that everyone else can get stuffed - you find reasons to view other people as somehow undeserving, and then what does it matter if you totally wreck their lives in the process of Governing only for your supporters? That's the Conservative approach and to my mind it is utterly immoral - because all people's lives matter.
But if you take the approach that Corbyn seems to be taking - of trying to appeal to only your core supporters, and not seeking to communicate with those who don't already agree with you, not seeking to understand their concerns, to see how you can come up with policies and a message that serves those people as well as your core supporters, then it seems to me that what you're doing is morally not much different from what the Conservatives have been doing for decades. (As well as being less practical, because there almost certainly aren't nearly enough Corbyn supporters to elect a Government in the first place).