The fad of email signature while responding to emails via phone was started by Apple iPhones as a way to show off.

I personally fail to understand signature of the types “excuse typos, sent via mobile device” annoying. What that really means is although I read your email with two minutes of your sending it but neither have the patience nor professionalism of drafting a proper response.

If someone wants to respond via phone/tablet how much time would it take to review and spell check before sending. Else it can wait till one can find a place to open his laptop and compose a response.