I don't mind bands/artists covering other songs, if they can do it justice - I think it is a great tribute to the original artists. Freddie's version of "The Great Pretender" is excellent and very fitting.
I think that any covered song could do well if it is done well - even one which would never have done well if Queen had released it. Also, getting to number one is as much (well more) about publicity than talent or quality.
If Mustapha, for example, was covered by say Westlife or Blue, mixed with some strange hiphop beat and promoted to all the teenie boppers then it probably would get to number one.