Yes, preprints are available in Dimensions and are included as one of the publication types (the other publication types are articles, chapters, monographs, edited books and proceedings) available in the "publication type" filter on the left of the Dimensions page (see screenshot). Preprints in Dimensions include arXiv, SSRN Electronic Journal, bioRxiv, ChemRxiv, PeerJ Preprints, Nature Precedings and others.
Many preprints are later published in peer-reviewed journals. Where it is possible, Dimensions includes a link on the preprint page to the peer-reviewed journal article that correlates to it (see below for example), as well as links to the original page and full text document. Links to original preprint documents are also featured on the journal article publication page in Dimensions where possible.