Depends on what you consider to be "support", but this report is pretty interesting and says something like 24% of US adults sympathize greatly with Mangione, and 63% have some non-zero level of sympathy for him. Outright approval for his actions isn't directly quantified by this poll but is undoubtedly lower than that 24% figure.

One interesting thing is sympathy for Mangione doesn't seem very strongly influenced by income level or level of education. The two biggest mediators seem to be political alignment and age. It seems the vast majority of US adults under 50 have a significant amount of sympathy for him, with only 28% expressing no sympathy at all.

https://www.cloudresearch.com/resources/blog/mangione-suppor...