CLOB vs Varchar2 performance in this case?
Pre-12c: varchar2 is limited to 4000 bytes. If you need to store more data in a field, go for CLOB.

From 12c: varchar2 is limited to 4000 bytes, or 32767 bytes after enabling Extended datatypes. If you need to store more data in a field, go for CLOB. With Extended datatypes, varchar2 is stored as a LOB internally, where "short" fields go to the table segment, while "long" fields go to the LOB segment.

Both types can be indexed (Oracle Text).

LOBs are designed to be written once, then treat as read-only data. DML operations on existing LOB data typically perform worse than on varchar2.

LOBs support deduplication (requires the Advanced Compression option).

Depending on the use-case, varchar2 can perform better or same. 

But based on what you wrote above (read-only data, column is not accessed or searched), CLOB should be fine, and you do not need to fiddle with Extended Datatypes or care about the limitations of varchar2.