Dressing up tends to mean that the person has put on nicer clothes. A carnival is a good time to "dress up," because you're going out and doing something special that you wouldn't do on a normal day. So you put on nicer clothes to look "dressed up" for the event. A lot of times, dressing up means much more formal clothing than everyday clothing.

Get dressed vs dress up To get dressed is a phrasal verb. It means to put on clothes. For example: I get dressed after I've had a shower. To dress up is a phrasal verb. It can mean to put on a costume, or to put on your best clothes.For example: I dressed up as a zombie for Halloween. I got dressed up for my job interview. (Source:http://www.learnenglish.de/mistakes/getdressed.html) See also: https://www.youtube.com/watch?v=EYE6ReLMBmI