The art style reminds me a lot of the original Binding of Isaac (and not just because of the flies and poop). Scroller or not, the norm for character control is definitely keyboard. Going with mouse only is a bit of a risk, but I think you've nailed it here. The smoothing is snappy enough to feel like the fly is doing what I want it to without be so sensitive that it stops feeling like a physical object with inertia. Strangely enough, I found the flight trail to be pretty useful in breaking up the monotony of restarting a run. You can swirl around and doodle until you have something to do.