You CAN register someone else’s trademark since there is no technical blocker for you to do that. However, it may cause you legal trouble. As in any other offline or online situation, the name may create a conflict with an existing trademark and such person may have a claim against you to cease using that name as ENS domain and if you don’t follow such a claim, a German court (if competent) may issue a judgment against you. The problem as of now (March 2023), is rather that there is no effective way of enforcing any such judgment due to the anonymous and decentralized nature of the blockchain. Best the trademark owner can achieve is to inform centralized platforms to red flag the respective ENS domain (or rather the respective token) as legally problematic and to contact the holder with a request to hand you the token/ENS domain and/or stop using it.