I was wondering when working on a portfolio should i focus more on mobile than websites on desktops? I say this because websites don't seem to be as UI/UX centric compared to an application or mobile app.
Mobile UI/UX is certainly a less developed area than Desktop, and faster growing at a faster rate.
It's importance cannot be understated.
However, the truth is - it depends on you and what you want to do.
Are you passionate about Mobile? Is that where you see yourself working? Can you relate to Mobile users over Desktop users? Can you understand the User Journeys of someone with a smaller screen and limited data better than the full-power of a Desktop? If so, work on a Mobile portfolio, and apply for Mobile portfolio-related jobs.
If you're just worried about getting a job full-stop, then as someone who has hired people, I would say that having evidence of both types is more useful to a UI/UX team manager.
Many teams have Junior and Senior levels who work on both Mobile and Desktop experiences, rather than just dedicated Mobile and Desktop teams.
Even if I was hiring someone as a dedicated Mobile UX consultant, having Desktop experience certainly isn't going to put me off. The ability to show an understanding for the needs of different Mobile/Desktop environments would only add to an application.
Are you a UI developer? Then I say concentrating on both are important. Because that is what you do for a living and people are going to judge you based on your website.
Most of the people today are lazy to open up a desktop and type in the URL unless they are finding it in the desktop. Most of the times when I get a url , what I do is to check the site in my Mobile. So if a mobile site is bad , the first impression is gone , and yes first impression matters.
When it comes to mobile UX make things simple, to make a no nonsense mobile version of a site is no big deal. Just try not to make it irritating for the user , he must be able to reach all the contents with much hassle.
You can concentrate less on UI and concentrate more on UX when it comes to mobile site . Even if it looks less good , it should not irritate the user - that should be the basic aim. But make sure you have a decent enough UI.
Desktop UX and Desktop UI
No excuses here , if you are making your portfolio you need to come up with your personal best both in UX and UI.
Check this site for example - http://www.rei.com/
They made a wonderful version of the mobile site , even with so many data. So when it comes to portfolio ( Since data is going to be comparatively less ) things are going to be easy , there will be zillions of example which you can learn from. Every other site today has a mobile version which has a decent UX and UI.
In all likelihood your portfolio will be viewed on a desktop.
There are very few recruiters who will look at it in any other form (it's unlikely that they'll be looking for it on a smartphone or tablet simply because, when they're out of the office they are visiting clients and not looking at candidate portfolios).
Similarly, there are very few employers who will look at a portfolio anywhere other than sitting at their desk focussing on how they will spend their staffing budget most wisely.
If you're a UX Designer then you're portfolio content will be the focus of the interaction and not the build of the site - If you can't code then you can't produce a responsive site now matter how well you can design one. You can just make sure that the case studies you show demonstrate your skills in designing for different screen sizes and formats.
Alternatively, if you're a UX Developer then you really want to be showing off you skills with all sorts of semantic tags, microdata, responsive CSS, media queries, etc.
So, as had been said in other comments here, it depends what job you're looking for.