Q: Will a manuscript published in a mirror journal of a reputed parent journal be considered less important?
The parent journal and mirror journal have the same editorial board, peer review, and aim. However, the mirror journal has no impact factor at present.
Mirror journals are a relatively new concept that journals and publishers are experimenting with to support the open access movement. These journals are fully open access and have the same editorial board and peer review process. In fact, the peer reviewers are generally not even aware of whether the article will be published in the parent or the mirror journal, so there is no possibility of bias.
To answer your question, as I mentioned earlier, this is a very new concept and therefore it is difficult to have a definitive idea of how universities and academic circles are viewing publications in mirror journals. If your university emphasizes the impact factor, it might be a good idea to submit to the parent journal. However, if that is not the case, publishing in a mirror journal would be preferable as it would give you more outreach. Moreover, more and more funders are mandating open access; thus, this decision on your part to publish in a mirror journal can be viewed in a positive light by funders for subsequent projects.