The words "existential philosopher" are not yet* as ridiculous as the words "inspirational poet". But they should be. First of all, both are pleonasms. All philosophers are existential, all poets inspirational. That's their business. It's silly for a poet to insist on the role of inspiration in their work. The same goes for the role of existence in the work of the philosopher.
*The Pangrammaticon has not yet been influential enough.