If we go by the decade idea of "eras", which is the weakest? The 90s and 70s are considered the two strongest and most competitive ... Which decade or decades are the weakest? The 80s are often referred to as a weak decade but compared to some of the recent decades the 80s weren't that bad .... I'd say the decade of 1980 was about average for heavyweight boxing ...
I don't think you can group bad periods into entire decades. The 70s are considered a great decade but really only until around 76 (maybe not even that long). The end of the 70s wasn't filled with great heavyweights in the least. In an era like the 1990s, which is considered great, right in the middle of it, the three heavyweight champions were Frank Bruno, Bruce Seldon and Frans Botha (before Botha's drug test came back) - I have a photo of all three in one magazine smiling holding up their belts. Somehow, they poppped up when 46-year-old Foreman refused to rematch anyone, Holyfield had a bad heart, Tyson was just out of prison, Bowe was falling apart. Lewis still hadn't fully come back from the McCall loss. So, it depends. There were plenty of low periods in nearly every decade, even the best ones. The weakest would take some thought.
If all the "interim" and "regular" and "gold" and "silver" beltholders were the actual champions - Bryan, Helenius, Charr, Povetkin - I'd agree with you. But Tyson Fury and Anthony Joshua are recognized as the top two beltholders right now, and Fury is the Lineal Champ. Fury and Joshua are not the weakest champs in boxing history by a LONG shot.
The 1930s was the first strong decade imo. It already sank again by the 40s though. Joe Louis was in complete control, even after he was past it, and the division didn't appear to be building back up again until the arrival of Walcott and Charles near the decade's end. The 50s was a semi-weak era. Although Charles and Walcott were still factors in the early 50s, Marciano and Patterson were the only great heavies of the 50s, and no one to fill the gaps. The late 50s and 60s was one of the division's greatest eras, blooming with nearly 20 great heavyweights. The 70s is the greatest imo. I've already said on the 80s, and although the 90s lacked a lot of skill, there were a lot of great fights. It all declined after that. Roughly the 30s through 90s, my favorite eras, are the strongest. Before and after tend to be the weakest.
The turn of the century had some horrendous plonkers fighting for the title. Big clumsy oafs with no athleticism swinging away with their chins in the air while tripping over their own feet, or scrawny weaklings blatting away with telegraphed arm punches from endless clinches. Louis's era wasn't much better, but had the likes of Louis himself to elevate it. Past the 70s I'd say the period of 2000-2010 was particularly weak. Lots of fat, low-skilled punchbags cluttering up the top ten. This current era isn't particularly strong either. 70s and 90s were killer, and the 80s were underrated.