Bonaparte not only had more power than Cromwell but had also conquered imperial land and broken the Holy Roman Empire. The title gave him legitimacy over that, however paper thin, while also being new enough to the French that it wasn't associated with the royal regime nor the directorate.
Cromwell accepting a royal title would have lost him power. Best he could do was have his son selected as next Lord Protector.
Napoleon's title was not adding anything to the "legitimacy". It was adding something to his legal prerogatives within France and made him
formally equal to the top 2 European rulers, which would not be the case with a royal title (and, well, of course, no associations with the old regime). The "thickness" of his <whatever> was defined by his military power, which was anyway but "paper thin". His empire had nothing to do with the HRE because he was an emperor of the French and influence in Germany was formally defined by a different title (BTW, most of the "imperial land", territories to the West from the Rhine) had been conquered by the French Republic, not Napoleon).
Cromwell would lose nothing in the terms of power (Army is there and Parliament is or will be dissolved) but accepting a title would guarantee succession of his son without any kind of election.