Short answer: Yes. You can use capital letters in the display URL.
An expanded text advert has one line for the Display URL. It’s shown in green on the ads. It consists of your domain name and two optional 15 character fields called the path fields. Google help on expanded text advert path fields is here.
The domain name is taken from the page the visitor eventually lands on when they click your ad. Google calls this the destination URL.
Google might show the domain name in lowercase letters even if you’ve used capital letters. They may also show it with or without the www in front of the domain name.
Google’s thoughts on the Display URL are here.
The image below comes from Google's help section on expanded text advert paths.