Oh my god, you might be the greatest in something, still the question might be - how far ahead are you from the others, actually?
There's much more in the Stones than what's known to general public, so to say. Like you got a couple of songs that made them "greatest rocknroll band in the world", but most people don't know much more. And there's first class stuff all along the way. That's why you can call them underrated.
Ringo sure didn't think the Stones are underrated in the eyes of a hardcore fans. As a hardcore fan, I see a huge distance between the Stones and other bands. But if you tell someone you love the Stones, sometimes you hear "Hm, Satisfaction really rocks, I like it, but it's kinda old, isn't it. And I can't believe they're still around. Do they got anything else worth listening to...?" Well, that's a bit exaggerated, but still, you get it. The Stones are underrated - by majority, that is.
No course in logic is needed here really, I thought this is not so hard to get even without it