If you are new to WordPress, or you are a designer looking to make a living working with WordPress, I would highly recommend that you pick a niche.
Be a Specialist
By specialising in one niche within WordPress, you are in essence showcasing that you are a specialist in this area, particularly if you have a few client examples from within this area.
If a client is looking for WordPress Designer in Google, and they find your website, which specialises in their sector, and shows examples of other companies within their sector, you will have a good chance of being hired.
You could charge a premium because of your understanding and insight into their sector.
There are great marketing and SEO benefits to picking a niche, at a fundamental level; If you talk about one specific topic on your website, then Google will deem you highly relevant; and reward you by sending you visitors, the same applies across your Social Media Marketing.
Hopefully I have convinced you that picking a niche is a good idea, however on designbit.co.uk I do not market to a niche, but I do have a list of sector specific websites at the ready.