In the peer-review form (How does the JMIR peer-review form look like?), you will find a question on possible transfer to another journal.
JMIR Publications has a system of cascading peer-review (or "portable peer-reviews") (see FAQ article for authors: Why has my article been transferred to another journal (or a transfer has been suggested), and what are my options?).
This means that as reviewer you can suggest a transfer to a JMIR sister journal if you feel the paper is better suited for another journal of the JMIR family (Which journals is JMIR Publications currently publishing?).
This can be a result of the topic of the paper (if it is narrow and of interest to specialists only, it may be better suited for a sister journal), or impact (or lack of impact).
JMIR Publications has "flagship journals" (e.g. JMIR and JMIR mHealth & uHealth), which have high impact factors (What is an Impact Factor?). If you find a paper in principle publishable, but not strong enough for the target journal, please suggest another journal, instead of recommending a "reject". Often, papers with "thin" results can also still be "rescued" by suggesting that authors convert this into a proposal/protocol/formative research paper and transfer it to JMIR Res Protoc (Why should I publish my protocol or grant proposal?).
The peer-review form also contains a question on suggested impact on a 1-10 scale, which will further help the editor to make a decision on cascading the manuscript.