I actually thought that was a feature?? The flying enemies can't be got if you let them come at you from above, and similarly you can't let the little guys (my daughter calls them the 'bunnies') come at you low.
I still don't agree with you. Mouse aiming is great for shooter games but I have my doubts about using it with hack and slash. As an example, you have the amazing Crawl, which uses the same principle of horizontal interactions but with vastly superior gameplay than DvsD. I grant you the hit boxes in the original version of DvsD have been rearranged to make it easy to hit monsters even vertically. So please give the latest versions a try.
As always! Thanks for the comment.