Is it good to use .NET datatypes in SQLCLR?

No, it's always recommended to use SQL Server datatypes for SQLCLR code as it implements better integration. Example for "int" datatype in .NET we can not assign NULL value it will crash, but using SQL datatype Sqllnt32 NULLS will be handled. All SQL datatypes are available in "system.data.SQLtypes", so you have to refer this namespace in order to get advantage of SQL datatypes.