Hitler is far from unique in pushing one ethnic group out. Throughout human history that has, sadly, been common.
Percentages are important, but Hitler was responsible for the death of millions. Absolute numbers matter too.
I looked more into the history. (Your cite doesn't give 600,000 for the figure killed by Cromwell, but for the whole civil war. In fact, the author ends "One wonders how many of the c. 600,000 victims died during Cromwell's campaign." The cite there is to Michael St John Parker, which I can't find.) I'm still hunting for reliable estimates of the population decline. Houston and Houston "Population history" isn't available online. The writers in Ohlmeyer (cited below) don't seem to put up a headline figure. William Petty's estimates aren't modern enough to be useful, although Petty was a serious writer. Note two points. First, population decline isn't the same as causing deaths, especially in a young population. Iraq after 1990 saw "missing population" of up to a million due to sanctions, but this is probably not because a million people were directly killed. You also have to account for declining birth rates and emigration. For instance, marriages fell sharply at the start of the civil war period. Other deaths were due to famine and starvation. Again, these are appalling, buut they are not the same as murder in death camps.
More importantly, these are figures for the entire 1641-1661 conflict. But Cromwell was only there relatively briefly. He didn't start the conflict, nor finish it. He became a shorthand for the whole English policy of the period. That is not necessarily fair to him.
None of this makes Cromwell innocent. (Still less does it make the English leadership as a whole innocent.) But the Hitler comparison are still overblown:
* Cromwell took part in a brutal colonial civil war, which devastated the Irish economy and led to famine and disease. He was responsible for massacres at Wexford and Drogheda.
* Hitler committed deliberate, systematic genocide against at least two ethnic groups, setting up scientific-industrial systems of mass murder which killed millions.
These are not the same.
Things I read while learning more about this:
https://academic.oup.com/past/article-abstract/195/1/55/1523...
Rai, M., 1993. Columbus in Ireland. Race & Class, 34(4), pp.25-34.
Ohlmeyer, J.H. ed., 2002. Ireland from independence to occupation, 1641-1660. Cambridge University Press.
Update. The most serious, detailed estimates I could find are:
Lenihan, P., 1997. War and population, 1649–52. Irish Economic and Social History, 24(1), pp.1-21, available at
https://search.proquest.com/docview/1292941669/fulltextPDF/A...
These give an estimate of population decline of 15-20%, mostly due to famine and plague (which the war was instrumental in spreading).