Using 'do somebody no favours' suggests someone's actions may harm their reputation or image. Showing up late to meetings does you no favours with clients.
Pro Tip 2/3
Not Just Appearance
The phrase doesn't only relate to physical looks but also behavior or performance. Complaining about work does you no favours with your boss.
Pro Tip 3/3
Negative Outcome
It implies that someone's actions have negative consequences, even if unintended. Slacking off on the project does us no favours for meeting the deadline.