To "make ___ look bad" can mean two different things:
- To do something stupid or embarrassing which makes the people who are associated with you (your boss, your classmates, your family members) ashamed:
Sit down and behave yourself! You're making us look bad.
- To do something so well that other people's accomplishments don't seem very impressive in comparison:
Slow down! You're making me look bad.
The first one is usually used seriously. The second one is usually used as a joke. In the example at top, the speaker says it as a joke to show that he's impressed that his friend is selling so much.