Is Whisper-Mumbling a subgenre of pop music?

What does that have to do with the topic?

You aren't wrong. Plenty of pop careers have been made with autotone (cough..ashely simpson...cough). but even though Eilish has used autotune from time to time, she doesn't need it, (and I would argue she sounds better without it). Her natural voice has great sound, timbre, and range. That "mumbling" OP is referencing, is actually an indicator of how good a vocalist Eilish really is. Plenty of trained sopranos can hit that e3/e5 at the top of their lungs (and it is impressive when they do it), but very few can hit those notes at barely above a whisper. Eilish can. Not only that, she can go from e5 (mezzo soprano) all the way down to a g4/5 (baritone). That is impressive range and she can do it at barely above a whisper. There isn't a single autontune pop princess out there that can sing like Eilish can, and that is why she is famous despite the fact she likes to go onstage dressed in a tarp with her hair painted green..