I think I may have gotten in the higher teens or maybe early 20s once? I played for another hour or two while listening to a book yesterday but eventually I gave up. The loop is fun and I enjoyed it. But my average death was probably before 20 at that point, usually I have to use some supers to keep myself alive and they are too expensive to get more of.
I might also just be picking my mutators poorly. I was doing the level 3 damage one, level 3 half health on some chickens one, and the third one just picking something. I think I usually tried the coin burst after boss or life gain after boss one, but it seemed to me once you start losing lives you're basically gonna die so having extra lives or slowing chickens down on the last life etc didn't do much.