Simplicity vs complexity

Always try to use the simplest possible UI that still properly conveys the game state and lets the user interact with it. Cutting down on the simplicity beyond that point just worsens gameplay. By simplicity, here, I mean ease of use (interaction) and legibility (conveying information). Make it so that it takes as little mental strain as possible to look at it and extract all the information at a glance, and make it so that issuing actions is also as effortless as possible.

On mobile, you usually have little screen estate to work with, so lots of stuff is hidden in contextual menus that only pop up when you touch some object, and since you don't have multiple mouse buttons or modifier keys (ctrl, alt, shift, etc.), gestures are also quite important, especially drag&drop, sliders, and tap vs. long press, scrolling views, etc..

If you can't make an easy to use or read UI, then you need to try harder or simplify / rethink your game design, so that you can find a design that allows you to make a good UI. Share a screenshot if you want concrete feedback on your current UI, but I think you phrased the problem the wrong way. Immersion comes from emotional investment, caused by things such as beauty, familiarity, and stakes. As long as the UI meshes well with the game's look (like having a wooden-looking UI when playing elves in an RTS), it does not reduce immersion. However, when it is complicated to interact with, then it breaks immersion, but the same also goes for clunky controls in a platformer. If any part of the game is hard to interact with, or does not mesh with the overall visual style, it will break immersion. Minimalistic vs. complex UI doesn't really impact this nearly as much. You can have clunky minimalistic UIs and you can have really smooth complex UIs. So either your UI isn't aesthetical enough or not functional enough if you encounter problems.

The art of UI design is to find creative ways to express a lot of information in a way that can be absorbed at a glance, and to find ways to issue commands as effortlessly as possible.