I was just reading somewhere that cable is (was, i guess he's dead now) as smart as Reed Richards. Is that accurate? Isn't Reed supposed to be the smartest man in the Marvel U?

He is meant to be Reed Richards smart. '' Cable possesses an intellect that may be on par with that of Reed Richards, which may be superhuman. He is also in possession of an education provided by the Clan Askani, which is far in advance of 20th – 21st century Earth-616 conventional education, especially in the sciences'' stated from marvel. 

"Smart" is a really vague term. 
 
In terms of information, Cable is probably on par with if not surpassing Reed just by virtue of his time travels.  It doesn't matter how brilliant Da Vinci was, if you pick up any science major college student with a 4.0 GPA and common knowledge, they would make Da Vinci look positively "stupid" with respect to facts about the physical world, science, etc.  And book-learning is a large part of intelligence.  People who absorb large amounts of information are deemed "smart' in some respects.  At one point, Cable's power was the "infonet" possessing the ability not only to access all connected information but process it and interpret it and control it, which is a pretty advanced intellectual feat but well within the realm of what is expected of telekinetics who "manipulate objects at a molecular level" (a concept I always found absurd for humanoids but nonetheless that's the canon).  TKs like Cable can perceive, understand, and manipulate all the molecules in a single object at once which is an incredible raw processing feat. 
 
In terms of tests or innovation, I suspect Reed would have greater baseline talent but once again nullified by Cable's time traveling advantage.  That is to say, if the need is for an invention and you ask Reed to "invent fire" he may on his own invent 3 different processes from scratch whereas Cable might only invent one or two BUT Cable comes from a future where 5 different processes are common knowledge already.  It's like taking an aptitude test knowing all the answers already.  Another way to put it, if Reed was given all the same time travel advantages, he probably wouldn't have same level of raw processing (TK mind) or technological advantages (cybernetic enhancements, the infonet, etc) but he'd be as creative if not more so than Cable, but as it stands Cable has answers from the far future. 
 
The one place were Cable falls down in practice is that he's less apt to use such future knowledge in a broad or public sense even if he knows of it.  Reed has a similar "prime directive" where his technology tends not to stray from the Baxter Building but for him everything is a matter of his own innovation in his own timeline whereas Cable's "temporal prime directive" is a bit more restrictive (although Cable, as a rule, is a pretty reckless time traveler). 
 
Back to the original question, in broad strokes I doubt he's ever written or thought of that way but if you break it down anyone from that far in the future with those abilities could plausibly be considered as smart- if not smarter- than anyone from our comparatively primitive time.