Grand Funk, however, went more pop as the 70s moved on.That song doesn't ring a bell to me at all. Maybe if I heard it, I'd know it, but I don't know it by name. At least for classic hits, familiarity is important. Part of the idea behind playing older music is that the target audience knows every song you play. Relatively obscure or regional hit songs don't pass that test.
All but a couple of the most popular Fleetwood Mac songs today were relative stiffs in their time. So, the simple answer to your question is yes. I don't know, however, if it was a matter of people not liking those Fleetwood Mac songs when they were new so much as it was a general lack of exposure at the time. Fleetwood Mac tended to be released by the album rather than by the single, which prevented it from getting the airplay on hit music stations it might otherwise have gotten.
Grand Funk Railroad is an example of a group that was hated by critics but developed a loyal following. In the 1970’s, it was the only group to ever outsell The Beatles at Madison Square Garden, and that record held at least 10 years. Over time, critics became less harsh on the group, and it remains popular with rock fans all over the country, including some who probably were unfamiliar with the band when it was new. You don’t hear Grand Funk too often on classic hits, but you can still hear the group all over classic rock.