Moral theorizing is irrelevant. Trademark law is the only relevant factor here.
The legal question under US trademark law is whether a reasonable consumer is likely to be confused by the name and erroneously infer that some sort of relationship or endorsement exists between the two companies.
As Tesla is a rather obscure figure among the general public, my personal view is that average consumers will not be confused and the Nikola Motors name does not infringe Tesla Motors' trademark. However, that is far from clear-cut would be subject to endless dispute in court, probably with both sides producing survey and focus group results promoting their interpretation.
They should name it Musk Motors.
But Musk isn't a Tesla family member or anything, right? Morally speaking, what gives Musk the right to use Tesla's name as an homage, but somehow this other guy doesn't have the same right? If Musk can call his company Tesla, somebody else using the name Nikola should be fair game.
It's probably not the wisest naming choice in the long run, but hey, people are already talking about it, so in a sense it's something that will get them some attention.