Hmmmm, it depends upon how you want to look at things. Sammartino's style would probably never be able to get the job done today. But, you have to remember that pro wrestling as a pretty different animal 45 years ago. Back in those days, you didn't have guys usually going to the top rope, you didn't have guys doing springboard dropkicks, superplexes, powerbombs, suicide dives, chokeslams, moonsaults, ladder matches, and so on and so forth. Most wrestlers didn't walk around with Herculean physiques, perfect tans, most didn't get their body hair waxed. Basically, the biggest wrestling stars didn't have to look like a cross between a Hollywood dreamboat and a Greek god.
Sammartino brought what he had to the ring, which was a legitimate physical toughness and freakish physical strength. The fans of the WWWF went wild for the guy because he was a tough, no frills Italian-American that kept their attention. Sammartino was offered the NWA World Heavyweight Championship during his first run as WWWF Champion and turned it down. Not only because of the intense schedule, but because of the money he was making doing exactly what he was doing. He was making more money as the WWWF Champion than he would have made as NWA Champion and was working only a fraction of the schedule. Sammartino drew big money, people wanted to see him do his thing.
So, was he overrated or underrated? Hmmm...well, I probably have to say underrated overall. For his era, Sammartino was a pretty good wrestler. If you try to measure him by using today's standards, he's not all that hot at all. But, comparing wrestling today to wrestling 45 years ago is like comparing Vin Diesel to Lawrence Olivier.