I suppose there are two sides to an app for server games - one as a remote viewer (which I think is a valid proposition by itself, even without a minimap) and the other allowing remote interaction with the server game (possibly being a more complex undertaking?).
Yes, allowing remote interaction would be a very useful thing, in fact, for players to undertake management tasks without having to be beside a full-blown desktop client. This might well be a fairly complicated undertaking, but it ought not be beyond the pale in principle. The issue is whether there are enough skilled Simutrans programmers ready to implement this feature, I think, rather than whether it is feasible in principle, and that will depend in part, but not entirely, on whether enough people think that it is worthwhile.
As for the map, I wonder whether it would be possible for the server to generate some sort of simplified version of the game map, showing just land and water areas, town hall locations and town names, and for the statistics to be able to identify stations by co-ordinate such that they might then be shown on a simplified version of the map based on that just described to give people something of a graphical representation of the game world without all the computationally intensive detail required for the full game. It would be useful to get some idea, for example, of where a rival has built, or where graphically on the map that problems on one's own network have occurred.