Yes it’s a very reasonable start to get experience. There are generally lots of artists, designers etc who feel they need a website, and you can generally find people in this position through your own network of friends. If it is people in the creative sector you’re talking about, the other advantage is that generally they want something unique and interesting, so it gives you a chance to try various new things.
The major downside is that it’s unlikely anyone will have much money to pay you. It’s extremely unlikely you’re going to be able to sustain yourself. Portfolio websites are a specialist niche product for a tiny market. They are most needed by people starting out, who have little spare money, and they are most often built by friends of friends. People who are successful (and people who’ve graduated to running an organisation) also need these things, but by that point they also normally have larger creative networks and can lean on very experienced & highly skilled connections.
Also note that WordPress is probably overkill most of the time as most of the people you’re building it for likely won’t touch the site once it’s built.
Edit: Note that this is literally how I started out (albeit MySpace pages before WordPress sites), I was a designer/illustrator, started building websites, went from there.
Edit edit: you can make money to sustain yourself here, but you generally need to be really fast, very accurate, and very good at communication, politics, and selling yourself. It is much easier to be a freelancer after you’ve worked for a few years in non-freelance jobs, YMMV