Email is a text medium. Many email programs display the received emails in the font the user of the program sets and ignore any formatting (like font, font size, color, etc.) Unless you know that the recipient is going to be able to display the non-alpha-numeric (lower 128) stuff, don't bother. (Some email programs mess that stuff up to the point that it's unreadable and some people with sophisticated email programs filter out anything that has non-alpha-numeric characters in it. In either case, they won't see your emails. The same problems occur with HTML emails, even though they're plain old text.)
Whether you can change the font in the app, or in a part of it like the signature, is up to the email app, not the phone. Many Android email apps allow you to add a file as a signature - design what you want as a signature in a graphic format (jpg, png, etc.) and use that. Most email programs can display graphics (even if many people have their programs set to not do so - at least they'll be able to read the text of the email).