This is, of course, a matter of personal opinion. There is no way to objectively state whether they should.
My own view is that many of them have a moral right to return to their land. However, that is not the same thing as saying that they should be able to do so. Too much time has passed and too many problems would be caused by granting a blanket right of return to all of the Palestinians who once lived in Israel.
I would argue that the Palestinians do, for the most part, have a moral right to return to the land that they once lived on. When Israel was created in 1948, there was no morally good reason to throw the Palestinians off their land. Of course, you can argue that some of the Palestinians left voluntarily, but running away from a war is really not the same as truly leaving voluntarily. Because Israel had no moral right to take the Palestinians’ land, the Palestinians have a moral right to have the land back.
The problem is that there is no plausible way to make this happen. The amount of hatred that exists between Israelis and Palestinians means that there is no way that you could peaceably absorb large numbers of Palestinians into Israel. Therefore, I do not think that they should be able to return. I think that it is a moral tragedy, but it would be worse if they were allowed to go back.