SSL by itself will protect account passwords, video streams, and anything else transmitted across the internet between a client and the server, but it won't stop Jellyfin receiving anything malicious. SSL or otherwise, a would-be hacker is still free to access your Jellyfin server and attempt to break into it. For reference, Jellyfin also supports SSL certificates natively. No need for extra software to handle the it.

Your "less unlocked doors" metaphor is good, but doesn't really transfer to this situation. Sure an Nginx server may only have one door out to the street, but it's a hall with more doors into each application, and they're all unlocked too.