Depends entirely on the instance, Lemmy is just a piece of software.

Depends on the instance.

lemmy.ml, dbzer0, hexbear are all ok on TOR.

lemmy.world is questionable

I am not aware of any instance that is hosted only on Tor, but you can use Tor while logging into any instance to remain more anonymous... depending on the kind of credentials they ask for when you sign up.

I have no problems with it.

Make the question(s) more interesting:

• Are there instances which offer users the option to access their user UI+API via tor hidden services?
• Are there instances which primarily exist as tor hidden services, and thus, federation traffic itself must go through the tor network if an instance wants to federate with them?
• Are there clear net instances which would be willing to federate with the hypothetical tor hidden service instances mentioned above?

There are no onion addresses on threadiverse: no.
Some even use cloudflare tor disabled: no.