No. You only need citmail if something other than Citadel is the primary MTA on your system. If you've got Citadel and other mail systems running in parallel, then citmail is one way to get Citadel's mail routed in.
However, it's not the only way, and certainly not the best way. If you are using an MTA such as Postfix which speaks LMTP, you can have it connect
directly into Citadel that way. Look for 'lmtp.socket' in your Citadel directory. That's where you tell it to connect.
Also keep in mind that citmail is nothing more than a simple LMTP client that does exactly that. So if you're using something like Sendmail that doesn't have LMTP support, you can use citmail to help out.
All this is irrelevant, though, if Citadel is the only mail system on the host. If that's the case, simply shut off all other MTA's and let Citadel pick up incoming connections on port 25.