Tyson went the distance with bums like Mitch Green. The jab from tall fighters like Buster Douglas and Lennox Lewis were Tyson's Krytonite. His defense against bums was awesome in the mid '80's I agree. A prime Tony Tucker, are you serious? Larry Holmes as an old man. Berbeck and "Bone Crusher" Smith along with tiny little Spinks were great exciting fights to see mediocre fighters get knocked out.
If Tyson fought a decade earlier against a prime Foreman, Ali, Frazier, Liston, etc, he would have had serious problems. Even a prime Gerry Cooney would have given him fits. The mid 80's was very weak for heavyweight boxers so he came around at the perfect time. Not trying to shit on Tyson as he was my favorite boxer growing up but even he has admitted he was not close to the best of all time in interviews. The fact that he is honest and humble makes me respect him even more.
Its hard to say the mid 80s was "weak" in the sense of talent level. Its hard to produce any solid rationale for why that would be. More realistically, the HW division was regarded as weak simply because there was a massive decline in household popularity before Tyson came along and (through brilliant marketing employed by his original management) revitalized viewing figures and became literally the 2nd most recognizable athlete in the world next to Ali. One of the reasons promoters from that era talk about is the lack of personalities. The golden era isnt just marked by great boxers - it was marked by big personalities and big rivalries.
I agree Ali and Foreman would've given tyson trouble. Ali mostly because he was durable, his most underrated quality that allowed him to have a 2nd career Not so much Liston and certainly not Cooney. Cooney has been dropped by guys who aren't even known as punchers.
While i think most people overrate Tyson because of the success of how he was marketed, I think you on the other hand are underrating him in a bit of an exaggerated manner. He would do well in any era, because he was such a physical specimen and because of the systematic development D'amato put he through.