Please don't add that to the Web.config. The webapp config file used by xsp (typically with a .webapp extension) and the Web.config file are not intended to be the same thing and combining them might not work with future versions of xsp. From the xsp man page:
WEBAPP FILE FORMAT
The format of the .webapp files used for --appconfigfile and --appcon-
figdir is:
<web-application>
{appname}
{virtual host for application}
{port for the application}
{virtual directory in apache}
{physical path to aspx files}
{true|false}
</web-application>
You can include any number of <web-application> nodes, one per ASP.NET
application to run.
There’s a sample.webapp file in the XSP samples directory.
If you don't want to write a webapp config file, you can use (with an unmodified Web.config):
/opt/mono-1.1.13.2/bin/xsp --applications '/:/var/www/mojoportal/mojoportal/' --root /var/www/mojoportal/mojoportal --verbose --nonstop &
BTW, I removed the --port option you had since 80 is the default. I also removed the --https option you had because I don't think it will do anything unless you use some of the other options to specify the cert/key to use.
Hope that helps,
--Dean