Spassky was pretty good but yes, he was probably less strong than the other Soviet World Champions.
That said, he was the very first World Champion who mastered and combined both dynamism and positional play, the reason why his style was called "universal". After Reykjavík, it seems that he lost the ambition in chess, although he was still victorious in the 1973 USSR Championship (probably the strongest ever) and Bugojno 1978.
I'm surprised no one has mentioned Spassky yet. His level of chess in the match vs Fischer was embarrasing. Even before that he was destroyed in the USSR Championship, finishing 8th or so. After Reykiavik he never returned to the elite. He didn't create theory regarding openings. To be fair, he was a very strong calculator and played some excellent games i the lates 60s. But when we speak about «the weakest» world champions, undoubtedly Spassky is among them.