I wouldn't blame Unity for this. It's perfectly capable of running games efficiently on mobile. Problem is people either don't know how to or don't care to optimize their games performance.

Kind of by definition indies don’t have the resources to optimize their games as much as a major studio.

Sure, they're more limited but Unity actually has very good and accessible profiling tools included. It'd be easy for most developers to get quick wins if they've never optimized their game before.