Yes, people at the time said the league was weaker. There's a reason that Original Six nostalgia really hit its stride in the early 80s. Many hockey people remembered that era and knew that something had really been lost.
Arthur Therrien, junior coach who had seen the NHL since the very beginning,
said in 1974:
...the scientific game has disappeared. Especially since the first expansion.
Clubs of today, I dare mention Oakland, Los Angeles, Vancouver, and New York Islanders, are fundamentally mediocre. I would also dare say that in the National Hockey League, hundreds of players should be playing in the minor leagues, they are completely lost when they meet a team of the quality of Canadiens, Boston Bruins, the Rangers or the Chicago Black Hawks.
These young players chase after the puck, they don't carry it, they get rid of it as soon as they receive it. They don't know any better because they haven't received the training required to shine in the National Hockey League.
Jim Coleman, a columnist who had watched the NHL since the 1930s, spoke for himself and referenced the general opinion when he
wrote in a 1981 column:
It has become fashionable among oldtimers to deplore the overall quality of the players in the NHL today. Unquestionably, the 13-year expansion from six teams to 21 teams has permitted many defencemen of only borderline ability to find employment in the NHL.
Coleman went on to defend goaltenders of 1981, and said the poor defence was making them look bad.
The horrible truth is that, today, there simply aren’t enough good defencemen to stock a 21-team league. There aren’t even as many good ones as there were in 1966.
I don't have any more links handy, but I can say that in the late 70s and early 80s, it was common for columnists to comment about the low quality of the watered down NHL and say it wasn't as strong as before expansion. Tim Burke of the Montreal Gazette was one, as he didn't rank the late 70s Montreal dynasty over the late 50s dynasty due in part to the low quality of opposition faced by the 70s team.
Some, including Coleman, were of the opinion that international play against the Soviets were the place where real quality was tested, because NHL competition of that era was too poor to measure the greats. For example, Coleman wrote in 1983 that Gretzky's scoring feats were impressive, but were compiled against a weak and watered down league, so let's put any discussion of him as the greatest ever on hold until we can see him in the 1984 Canada Cup.
The losses to the Soviets in the 1979 Challenge Cup and the 1981 Canada Cup really reinforced the idea that NHL quality had declined.
Apr 2, 1975 – Tim Burke wrote about Guy Lafleur and Maurice Richard
The Rocket last night was non-committal when asked how many goals he thought he would score in this period of talent dilution and league expansion.
“All I will say is that I would score as many goals as Esposito,” said Richard.
I’ll say it then. At least a hundred, Rocket.