Is it safe to upgrade iPhone 5C to iOS 9? I want to do it before iOS 10 launches and it becomes impossible to use 9. But I'm skeptical given the low-config nature of 5C.

Are there any known performance degradation in 5C if iOS is upgraded to 9 from 8? So far it is working good with 8 and I don't wanna ruin it. The usage is very basic, I have bought it for my Mom.

And, is it really necessary to upgrade? I mean, is it possible that some apps might not work in future because I have not upgraded?

Thanks in advance.
The iPhone 5C will run iOS 9 fine, as iOS 9 requires no greater hardware than iOS 8. My inferior iPod Touch 5th gen (comparable to an iPhone 4S, two years older than the 5C) became slow upon upgrading to iOS 8 but didn't get any slower when upgrading to iOS 9.

As always, do a backup before upgrading.