I've not heard that Lewis (note for other readers: that's David Kellogg Lewis, the mid-to-late 20th-century philosopher best known for modal realism, even though his dates don't overlap with Ramsey's) is regarded as "outright crap" by anyone of substance. But I'm not particularly well up on the internal disputes of philosophy. Who doesn't like him?
Supposing that indeed some people do hold his work in low esteem, I'd guess it's because of his modal realism, which is for sure a major bullet-biting exercise and might strike some as silly. (I suppose he might also be contemned in some quarters for "political" reasons -- e.g., he was a fairly outspoken atheist, and I wonder what Alvin Plantinga thinks of him.)
I majored in philosophy. The only full-time, tenured analytic in my department thought that 1) modal realism was ridiculous, 2) David Lewis was incredibly smart, almost as much so as... I forget who he thought was the absolute smartest in recent times, but I think it might've been Kripke.
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