A portfolio is just a fancy word for a bunch of domains.
Most of the domain parking services allow you to "customize" or "optimize" the template for your domain. You can change the keywords, the title and description, add text, change images, and other things. If you have a domain about cars, having pictures of boats or kittens may not work as well as it would it you have photos of cars. In the same way, if your ads are for toothbrushes and not cars, you are not likely to get many clicks. But when everything matches, then there is a good chance people will be interested and click on something so you get paid.
As it was already stated, most parking services do not allow you to do ANYTHING to get more traffic to your domains. The traffic must ONLY come from people typeing in the domain name, OR from existing search engine listings.
BUT some parking services are not so strict and DO allow you to put links on sites, or even pay for search engine advertising. But anything that can be considered "fraud" is NOT allowed, including buying "hits" from services that sell traffic. You have to read and understand the TOS and rules that your parking service uses or they may close your account and keep any money that was earned.