Migrate from an early 2015 MacBook Pro running Catalina to a 2019 MacBook Air running Mojave
This has already been answered here

Anyway you should be fine as long as you only import data, e.g. desktop, documents, downloads, etc.  I would not try to carry over ~/Library folder, as this may contain items that are only relevant to the 'newer' version of macOS.  You should probably separately export Apple Mail and re-import as well.

That being said... why don't you just first update the 2019 to 10.15 then directly import your time machine and not worry about incompatibilities?