The answer most people will give you: no, it is not particulary useful in embedded systems. 10 years ago people predicted that Java, which is very similar to C#, would become an important language in embedded systems. That has not happend. Java is only widely used in mobile phones / pocket PC applications, that are actually much closer related to PC programming than to the embedded branch. There are very few cases where Java is used for actual firmware. So I don't think C# will have a chance there either.

If you instead spend your time with for example learning electronics, it will make you a better, more qualified embedded sw engineer.

However...

Embedded folk like myself always complain about having to deal with useless PC programmers trying to make embedded applications. But we rarely consider the opposite: embedded programmers making useless PC applications. The result here are equally awful: on-chip debuggers, eeprom burning software, protocol listeners, configuration tools etc almost exclusively have horrible user interfaces. They are rarely stable or reliable programs, and they are always single-threaded, making the graphics lag out in case there happens to be other programs running on the PC (there tend to be...). These programs are often the result of embedded programmers meddling with things they don't know about. A skilled PC programmer would not come up with such programs.

An embedded programmer who can also make nice PC programs is therefore valuable. For this reason, C# or another desktop language is good to know in case you will end up making PC user interface programs for your embedded app.