It’s not directly related to the torrent or its content no. It’s more related to the potential bugs in Transmission that might be exploited to propagate viruses.

Since Transmission has to exchange data with un-trusted parties, before knowing whether the data is relevant to the torrent you are downloading, anyone could exploit bugs that exist in the parsing of these messages.

So running Transmission as a dedicated user limits what an attacker may have access to once they take control of Transmission through the exploit of known or unknown bugs.

Obviously, this user need to have many restriction in place as to prevent the attacker from installing malware permanently on the machine. And when you copy over data that has been downloaded by Transmission, you’d have to make sure it has not been tampered with by the attacker in an attempt to get access to the data available to your real account.

If you just use transmission occasionally, not on a server, I would not bother with it. Either use the flatpak version for some sandboxing and similar security guarantees as having a dedicated user running Transmission, or use an up to date version (the one from your distro should be fine) and don’t leave it running when you do not need to.

Running any service as a dedicated user with limited permissions is more secure than not. It’s not just transmission. This is a very basic method of increasing security on any running machine. https://en.m.wikipedia.org/wiki/Principle_of_least_privilege

It helps protect you because if the application in question is compromised in any way (or has a flaw, i.e. an accidental rm -rf /*), the only access it has is limited to the user it is run as. If it is run as root, it has full administrative privilege.