Is 5.5 hours enough time at HKG to see anything in the city and what can I do?

Can be done but it's tight.

Hong Kong airport has a very good train connection to downtown. However, it's pretty far away: 45 minute ride, train goes every 10 minutes, so I'd budget an hour each way. Train ride is comfortable and scenic.

If you need two hours of travel, 1.5 hours for security and getting to the gate plus half an hour of buffer (you never know), that leaves you 1.5 hours in the city.

You could get off at Hong Kong Central, walk down to the pier and than hit the star ferry http://www.starferry.com.hk/en/service. It's super cheap and a great way to see the city. Get off at Tsim Sha Tsui and wander down the Avenue of the Stars along the waterfront. Time permitting (and depending on interest) you can also walk down Nathan Road which is the main shopping drag brimming with tailors and fake rolex sellers. To return, take a cab to Kowloon station or the Ferry back to Hong Kong Central.

Things are a lot easier if you get an Octopus Card with Airport Express tickets upfront. Octopus is a swipe-to-pay system that's accepted on public transport and many stores (including 7-11 which are abundant). https://www.travelchinaguide.com/cityguides/hongkong/airport-express-pass.htm

There is a much more efficient alternative use of your layover time, which is the Ngong Ping Cable Car, also called the Big Buddha. It's much closer than the downtown, being on the same island as the airport. A short (and cheap) taxi ride will take you there, but make sure you get the right colour taxi. The taxis for downtown are not allowed to take you there. You may have to spend money to get a priority ticket for the cable car and avoid the line if it's a peak time. You should have time to ride the cable car, have a brief look round (seeing the huge statue of Buddha), and ride back. Getting a taxi back may be the hardest part, again thanks to Hong Kong's taxi colour coding scheme.