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.

If you watch "Frasier", you will see the leading woman character, Daphne, comes from Manchester (England) and sounds like it. Her brother has a role in various episodes but he has a strong London accent. Mufasa is a baddie and English actors are often called on to play baddies in Hollywood.