Yes, post a negative review and switch apps. These things happen alas. A few years ago I had this with a mindmap app (MindMaple): I had some questions and bugs to report. No response whatsoever. After two extra tries I gave up and switched to a different mindmap app (iThoughts) and left a negative review in the App store.
Interesting question, but totally depends on certain variables? Is it a free app that hasn't been updated in a long time? I would never expect a reply. Paid app, but not updated in a while? Some expectation, but since it hasn't been updated in a while, not very high. Paid app and recently updated, I expect a reply. Free but updated in a while? Solid maybe. As others have mentioned, if there is no reply, and you need one, move on to another app.
You can go to their website and submit contact form or email them.
If still no reply, you can mention in app review.
If you are facing any application issue, you can search alternatives. if it is just a feedback, dropping a mail or review would be enough.