It is one thing to debate whether EP is a good actor or not, but I'm sort of baffled by your confusion over why actors wear costumes. When you see a movie, generally those are actors wearing costumes -- not regular people wearing their normal clothes. For instance, in The Martian, those people aren't actually living on Mars, and those aren't real Martian space suits. They are costumes. Actors "play dress up" if you will so as to make movies and make a lot of money. Of course, not all actors wearing costumes are acting equally well.