Lovecraft was not a great writer, but he was great at imagining.
He writes with an awkward, clunky, affected style that does not flow well and can be disruptive to the mood he would like to build.
But the ideas themselves, the stories he wanted to tell, are in themselves so chilling that they transcend his less than stellar prose. A lot of horror and science fiction that we have today is influenced by Lovecraft. A lot of authors in these genres consider him a powerful influence.
It says something that authors like Stephen King, Brian Lumley, Robert Bloch, and others, have written stories set in Lovecraft's mythology. It also says something that these stories are usually better than Lovecraft's own.
Not a great writer, and yet a huge figure in horror and science fiction. He just understood what frightens people.