Is it allowed to perform DDL/DML operations on tables in the procedure?

Yes DML operations (update/insert/ delete) are allowed in the procedure also (DDL: such as create/ alter/drop/ truncate) is also allowed in the procedures.

In case if the procedure will get executed in parallel then DDL on permanent table is not recommended as there could be chances that one procedure has already created a permanent table and another procedure is trying to create a table then one procedure may fail.

However it is recommended to use DDL statements if they exist and transactions should be used elsewhere there will be a chance of failing a procedure.