You CAN design a separate UI for Mobile, but I suggest you look into responsive design. The UI on Ionic projects consists mostly of HTML/CSS elements, meaning you have free reign to design your application with every bit of flexibility offered by regular webdesign.
Check this to get started, it may not be 100% applicable for Ionic but it should certainly help:
https://www.w3schools.com/Css/css_rwd_intro.asp