There's no MimeKit API to quote a message, but it's not terribly difficult to do. The following code snippet is a good place to start (you may want to customize it a bit).
string QuoteMessageBody (MimeMessage message)
{
using (var quoted = new StringWriter ()) {
quoted.WriteLine ("On {0}, {1} wrote:", message.Date.ToString ("f"), message.From.ToString ());
using (var reader = new StringReader (message.TextBody)) {
string line;
while ((line = reader.ReadLine ()) != null) {
quoted.Write ("> ");
quoted.WriteLine (line);
}
}
return quoted.ToString ();
}
}
Hope that helps.