Not used Svelte but knowing how it works I guess MobX is simpler to debug. It has synchronous updates, so debugging is easy by just followig stacktrace. Not sure if Svelte is easily debuggable in contrast as it's compiled.
IMO You don't need mobx if you're using svelte. Use svelte stores.
but then you have domain code that depends on a view library, which defeats the whole point of having a store.