I have tried with PDI 6.1, Java 1.8.0_191, Oracle DB 12.2.0.1.0, JDBC Driver: ojdbc6.jar, and I can reproduce your problem. If you don't want to transform the TIMESTAMP columns to VARCHAR, you can use the step Select values, and use the Meta-data tab to change all the Timestamp columns to Date type columns (you only need to specify the columns you want to change) that way the Write to excel step works.
Maybe with a newer ojdbc library it might work directly, I use ojdbc6 because it was the one published at the time to work with Oracle 11.2 databases.
Regards
OS: Ubuntu 16.04 64 bits
Java: Openjdk 1.8.0_131
Pentaho 6.1 CE