Are all papers published by ACM indexed in DBLP?
I think this question can be answered via DBLP's FAQ, specifically: What are the criteria for dblp to index a journal or conference? In a nutshell,


DBLP has a venue application and review process that seems to be applied to each newly included venue,
During this process, a set of minimum standards for newly included venues is considered. One of these standards seems to be inherently fulfilled by conferences published in the ACM digital library: ''All full-texts of contributions should be available online (either openly or as a paid service) in an established digital library.''
There are no guarantees for (timely) inclusion: ''Please note that beside the eligibility of a publication venue, the workload and the resources at dblp are always a limiting factor for the inclusion of new venues''.