It is, but many times, I end up using Pocket or Instagram because the text is presented in a way that is simply unreadable for me, and neither Firefox's Reading Mode nor Clearly (when I use Chrome) help.
I think you mean Instapaper
Source on that? I don't know anyone who uses it that way, though it is one of their selling points
That's the only way I've ever used them and there are a bunch of send-to-Kindle services based on them so I feel like I can't be alone here. I'm not sure how you'd go about quantifying this though.
For what it's worth, iOS Safari has a directly available "Reader View" button that bumps you to a cleaned up view immediately with a single tap. The built in "Add to Reading List" is buried behind the share sheet.