If you are uploading your own logo to replace the default one, make sure your logo size is 186 X 97. If you upload a bigger or smaller sized image, it will automatically adjust the size which can make the uploaded logo appear distorted.
Follow these steps to change the default logo:
1. Go to appearance and click on "settings"
2. Upload your logo and resize the logo to 186x97
3. Click on "insert into post"
Don't forget to click on "save settings" under "Theme Settings".