Does MettleCI require the installation of any client tools?

No.  MettleCI functionality is accessed using the MettleCI Workbench, which is a browser-based tool. 

You may also take advantage of the MettleCI Custom Menu Items which can be installed into your Designer Client using the 'Integrated with DataStage Client' button within Workbench.  See this page for more details:  What does the Workbench 'Integrated with DataStage Client' button do?