SQL Mirroring Locking/Blocking Contention
1) If we utilize snapshot for reporting queries, and high volume OLTP
  transactions are sent from Server A to Server B, Will Report queries
  create locking/blocking contention, which will prevent Server B Mirror
  from receiving updates?


It will depend. If you are reading a page that has been changed since snapshot was created you are reading the page from sparse file. In that case there is no contention. But if you are reading from a page that did not change since snapshot was created you are reading from source database file. Which may cause contention at the file level since both the source database and the snapshot will be accessing the same MDF.

These are two good articles you might find helpful.


Database Snapshots by  Basit Aalishan Masood-Al-Farooq
SQL Server 2005 Snapshots by Andrew Calvett.



  2) If so, can I place the Server B Mirror database in Read-Committed
  Snapshot Isolation (RCSI), to prevent locking/blocking (while ServerA
  is in Read committed) ? Or does Mirror Database always have to be in
  same transaction isolation level as Principal ?


You cannot do that. As Mirror Database is in restoring state, you cannot make any change. Allowing different configuration goes against the principal or database mirroring where it guaranteed to have same configuration and data so roles of Principal and Mirror can be switched at any time.

Not related to your question but you might find this useful especially the comment in fact, you can’t execute this on the mirror, because the database is in the restoring state.

How to Enable RCSI for a Database with Database Mirroring by Sanjay Mishra