Most development shops will have someone else to design the UI for you. They'll give you the UI design, your responsibility will then be to implement it.
You'll be expected to design the UI yourself only in very small companies.
The problem lies in getting your career started. You will find that no one will want to hire you until you have shipped a product to end-users. For example all the job board posts seeking iOS or Android developers require that applicants have published Apps that are in the respective App Stores.
The way around that is to write a program all on your own. It's up to you whether you give it away free-as-in-beer, shareware, payware or open source. However you distribute this first application, it's important that you write it, that you do a good job writing it, and that you ship it to actual end-users.
That program will need to have a well-designed UI. If you cannot design the UI yourself, maybe you can find someone to help you.
When I was trying to break into Mac development for the first time, I wrote a simple vector graphic editor in C called CircleDraw. I brought the binary to job interviews with me on floppy disk, and put a hardcopy of the entire program's source code in a binder.