Yes, you can include them in the systematic review. JBI provides a tool to assess the quality of such studies: "Critical appraisal checklist for case reports" (https://jbi-global-wiki.refined.site/space/MANUAL/355599400/Appendix+7.4+Critical+appraisal+checklist+for+case+reports). However, let's note that Cochrane Handbook emphasized the limitation of case reports (SR of adverse events): "Spontaneous case reports or case series may assist in signalling rare and previously unknown events. However, for most Cochrane Reviews, these data sources should be used for scoping purposes only (particularly as they do not have denominator data to allow estimation of risks or rates). These spontaneous reports may guide drafting of the protocol when there is a need to choose relevant or important adverse effects as outcomes of interest." (https://training.cochrane.org/handbook/current/chapter-19)

We do a systematic search (for a systematic review) and retrieve only case reports as the final collection of articles. How rational is to title the study as a systematic review?

I think you should still call it a systematic review in the title, but you should state in the abstract that only case reports were found.