The Tor browser is based on Firefox, and like all major browsers, if it encounters a self-signed certificate, it'll put a big scary warning. There's no real security risk to them though. Self-signed certificates are necessary for any site that's not on the clearnet to have https, and https is required for Brotli compression.

If you click "advanced" then "Accept the Risk and Continue", you should be able to access Sayachan.

>>7
TLS is not necessary for Tor hidden services, it's just a redundant layer of encryption on top of Tor.
It's best to just to have the torrc configured to a regular HTTP port.