The Raspeberry Pi hardware isn't server hardware, and questions about the hardware are off-topic for Server Fault, full stop.
Questions about server software running on the "Pi" would seem to be on-topic, assuming that the "Pi" provides a deterministic compute substrate. A question about a given piece of server software seems to be on-topic to me, irrespective of the underlying compute platform.
Very rarely is the underlying hardware platform (especially in this age of virtualization) a factor at all in the correct operation of a piece of server software. The performance of a "Pi" may suck and the reliability may be questionable but software executing on a "Pi" is going to execute the same as on any other platform applicable to that software.
Edit:
I can get behind the idea that the "Pi" is such an unstable piece of computing hardware as to make it not a reliable compute substrate, though, too. This one is getting harder the more I think about it. I'd like to say that there is value in answering questions that end up with a root cause in performance analysis or assessing the underlying reliability of a platform. On the other hand if a large portion of "Pi"-related questions come down to resolutions like "Put the same code onto a more reliable platform and it worked fine, the 'Pi' is the culprit" then the "Pi", as a platform, isn't suitable for Server Fault.
I'm vacillating here. That's not good!