Technically, can we build the mirrors of lemmygrad or other lemmys in the dark network (Tor/i2p)?
Yeah, I think it wouldn’t be too hard to have a .onion address, although that would also make it easier for fascists to raid us which has happened a few times in the past, I guess. Still, having a .onion address only leverages some of the burden for Tor nodes in the process of requesting data, if you access Lemmygrad through Tor you’d still get the same anonymity advantages with or without a .onion address (as long as you have proper care with the use of the software).
As a developer, I think the Matrix protocol is not simple enough. I highly recommend the IRC server under the Tor/I2P network, which is simple and fast. In addition, I highly recommend a P2P text only twitter - twtxt (https://twtxt.readthedocs.io/en/latest/).