The question really is about ability rather than morals, and I guess its rooted in my perception that the Germans managed against increasing odds to carry the war on and defend frequently very well against superior numbers and equipment.
Ironically I think in the UK we almost want there to be a 'good' general who we can like because they weren't a Nazi, Rommel filled that position for a long time and now von Manstein seems to attract those sorts of comments as well.
Well, you can see from the comments here about people not wanting to select people from the Waffen SS or generals who were too involved in the Final Solution even though on paper morals shouldn't matter to this question, but obviously it does to alot of people.
The 8th Army fell in love with Rommel because he respected them and was out in the front with them while their generals were way behind the lines and often hanging out in Alexandra. The press liked him because he would invite Allied reporters to take pictures and talk to him. All this of course got back to Washington and London.
After the Final Solution came out with the end of the war the Jewish community for his actions on behalf of Jews various places during the war pretty much gave the U.S. and England the thumbs up for using him as as a Cold War symbol for 'the good German' to the West and to support German rearmament inside Germany. Of course his death made the story all the easier to make into TV shows and films.
But, generals popularity often get conflated with their abilities and it did very much suit the American and British governments interest in selling the story during and after the war that they beat the best Germany had. How effective was he as a general? Taking a public opinion poll on the matter isn't the best way to decide nor is focusing on what most historians at any given time are saying as they are far from unbias. One has to make ones own judgement from looking at the facts.