As I can understand from your question, you seem to say that "replace the downvote button with a '?' button". But that wouldn't really make any difference, except not showing a negative score. Practically, the downvotes would now be substituted by "?"s.
And if you are proposing that we need to provide a feedback after we click on that "?", then honestly, if I would have to write the reason for every downvote I cast, I would probably cast lesser downvotes, which is not at all healthy for the site. It discourages downvoting, and that's why it doesn't seem appropriate. And anyways, if anyone wants to, willingly, acknowledge and explain their downvote, they can comment on the post. Making it compulsory is not at all the right way to go.
And downvotes are far more effective in conveying the fact (to the OP) that the post has some issues with it. I don't think any of the help-vampires would take "?" seriously. So, all in all, it isn't a good idea.
If you get a few downvotes on any of your post, first check the help center tomdecide that whether your question/answer is even on-topic/acceptable (respectively) on this site. If not, the downvotes are completely justified. If it's on topic/acceptable (but it was still downvoted), then ask for the clarification from the downvoter in the comments and act accordingly to improve upon your post. At last, if you don't get any response, try asking about it in the chat and request for feedback. And at the end of the day, keep in mind that these are, after all, internet points and they are there just to indicate that someone thinks that there's something wrong with your post. There's no reason to take them personally :-)