The reason your reply address is not shown as the simplified address you have set (
firstname.lastname@example.org) in the email client is because the crazy numbers and letters, etc are actually containing the context of the original email for the Reply Mail Management (RMM) system in SFMC to correctly identify and correlate this reply in order to follow the associated rulesets for it. The final place this reply will end up is the address you specified after being processed in the RMM.
The process regarding RMM is as follows below:
Step 1: Email is sent and ReplyTo Address is created and sent in email header. This ReplyTo address is a mixture of the information you put inside of the RMM / Sender Profile and an encrypted string appended on to include context.
Step 2: When someone hits reply on the email, the replyTo address is filled into the 'To' field. This address is the one that includes the crazy numbers and letters. The 'Name' or 'Alias' of the replyto will be exactly as defined though, only the address will be altered.
Step 3: Once the reply is sent. This reply is then sent into the SFMC RMM system to be processed. After being processed, if it passes all parameters, the email is then forwarded to your designated reply address inside the RMM/Sender Profile.
To avoid this, you would have to avoid the RMM system and instead do a direct forward to that address. But do note that this removes all auto unsub and other 'benefits' that RMM allows you and instead would just be a direct reply to your email.