You can ban onion addresses if you dont want to trade with the peer. But due to the nature of Bisq they can create a new account/instance and trade with you again. Bisq is an alternative/option to centralised exchange, not sure how you expect peers to be screened?
If a peer is using a strange names or handle, you should not be continuing with the trade and open a dispute. It is mentioned in the pop up dialogue about this.
Treroorr groups? Doubt it, would be easier for them to sell via localbitcoins (localmonero.co would make more sense). More trade activity, higher limits(bisq is low for this reason), etc etc. You also have plausible deniability in a way, you are using bisq platform and have no idea about the other peer before hand.
You do have the option to make a physical cash deposit to the persons bank so that there is no link to you, but its not that common/easy on bisq. Some work is needed to make it as easy as localbitcoins/localmonero.co
A rating type system has been discussed before, im not sure the status of it. Probably not that high on the priority list.
Personally i dont mind the slowness of the fiat & crypto trades, im not a trader. Remember that the bitcoin money is well protected.
It would be useful as part of a future rating system, to rate the speed of a trading peer.
Can be calculated by how long one peer is waiting for the other peer, and it averages out over the peers trades. People can then decide if they wish to start a trade.