I think a sign message or the knowledge of it is a necessity for in-forum and off-forum Bitcoin-er, but many aren't knowledgeable about this necessity, I actually experienced difficulty try to sign a message from my mobile phone, till I tried it on my Laptop.
There is nothing hard to sign a message using mobile wallet, mycelium wallet can sign a message, coinomi can sign a message (but coinomi is close source which is the reason I do not recommend the wallet). Electrum can sign a message too, but only on desktop versions, its mobile versions do not support message signing even in the latest release.
Which mobile wallet did you use?
If it is electrum, it can not yet sign a message, only the desktop can for now.