Yes, if the conference article is received for reviewing from a standard journal for inclusion in a special issue. Elsevier publication issues such certificates for reviewers reviewing conference articles. I have received a similar certificate from Wear (Elsevier) for reviewing a conference article (submitted for Wear of Materials 2017 : 21st International Conference on Wear of Materials).
Nevertheless, some conference organizers dump bulk amount of papers to same reviewers. It is ethical to recognize their effort i.e. sparing precious time.