During the first part of the year I've been involved in a project where the scope included implementing/upgrading a bunch of standard Fiori applications to our on-prem launchpad, mainly in the area of HCM. A few months after go-live I can confidently state that some of these apps I like and some of them I do not find delightful at all (particularly the notorious My Timesheet V3).
In my opinion it would be great if the Fiori Apps Reference Library had a Google Play like feature allowing users to give a rating/brief review which can be seen by others as well. It would aid customers a great deal which app is worth to pick - given there is an alternative - and also SAP to assess how successful a product is and which ones require an improvement.
Or is there a platform/channel for this I am not aware of?
"given there is an alternative" - that's a big disclaimer. :)
I'm also not aware of a specific feedback channel for Fiori apps but general place for SAP products is influence.sap.com where using an S-ID you might be able to find some place to offer your opinion on an app.
You might want to ask this in Q&A, select SAP Fiori as a primary tag.