UI specific state (eg a flag to store whether the left panel is folded) should be as local as possible
MobX is not a replacement for local state. It's a useful mechanism for sharing state between parts of an app that need to share state - in the example of a left panel, if you want to be able to control that from outside of the component then you either need to be able to modify the state (eg MobX), or you need a callback that refers to the local panel state that's callable from outside (eg a Redux action), or you need a method that's passed around the app to where it's needed (eg a React context). If you don't need to control it from outside then you don't need any of these things and useState is fine.
Where MobX is especially useful is in things that aren't simply an observable value that need to be passed around - if you need to transform the value for other components to use it then MobX's @computed and @action are both _incredibly_ helpful.
In my opinion, the sharing of state in the manner you described leads to implicit coupling between components, which can be hard to hold in your head as the application grows.
MobX is not a replacement for local state. It's a useful mechanism for sharing state between parts of an app that need to share state - in the example of a left panel, if you want to be able to control that from outside of the component then you either need to be able to modify the state (eg MobX), or you need a callback that refers to the local panel state that's callable from outside (eg a Redux action), or you need a method that's passed around the app to where it's needed (eg a React context). If you don't need to control it from outside then you don't need any of these things and useState is fine.
Where MobX is especially useful is in things that aren't simply an observable value that need to be passed around - if you need to transform the value for other components to use it then MobX's @computed and @action are both _incredibly_ helpful.