No, it doesn't sound weird at all. That's because this is cinema, and cinema has its codes that we all understand. Just as it's a cinema convention for us not to question why cartoon lions talk at all, we don't question why two characters who supposedly have the same background should have different accents.
And here's the reason. Mufasa is good; Scar is evil. According to movie tradition, heroes have American accents and bad people do not have American accents. Mad, bad and violent people like Bond villians often have Russian accents. Evil scheming uncles who kill their brothers, like in Shakespeare's Hamlet, have English accents. That, as they say, is entertainment.