Can Bitcoin/blockchain be thought of as a decentralized time-stamping authority (TSA)? Is there true consensus in the Bitcoin network on a mined block's time, or just that the block was mined?
Yes, Bitcoin can act as a time-stamping authority. However, depending on your application the accuracy of the Bitcoin blocks may not be sufficient.
Nodes only reject blocks that are greater than 2 hours into the future, and blocks created earlier than the median time of the past 11 blocks. Therefore, timestamps may be off by several hours.
also see: How accurate is Bitcoin network time?
I don't think decentralized time-stamping authority does Bitcoin full justice, but yes blocks are timestamped and yes there is consensus on actual time. Albeit within a few hours margin. That even means that sometimes the timestamp on a newer block is further in the past than the timestamp of an older block.