I think that's an overstatement. "The Web of Fear" explicitly said in dialogue that "The Abominable Snowmen" was in 1935 and "over 40 years ago," explicitly dating the story as 1975 or later. "The Invasion" is explicitly said to be four years after that. The UNIT stories were clearly intended to be set in the near future, which is why they had things like a crewed Mars expedition, and of course Sarah explicitly saying "I'm from 1980" in a 1975 serial -- although that directly contradicts the Troughton-era references, since there's no way the entire Pertwee era could fit into less than one year.
So yes, "Mawdryn" was a gaffe. It wasn't the first one, but it was the biggest one. Before, we could mostly ignore the minor details you mention like calendars and car stickers as trivial errors while still believing the intent that the stories were in the near future (in the same way we choose to suspend disbelief about the fakey monster costumes and spaceship models and accept the underlying intent that they're real). That's a common enough thing to see in near-future sci-fi, for instance Gerry Anderson's UFO, which features the 1960s-est version of 1980 I've ever seen (even though it was made in 1970). So it doesn't have to be taken literally in-story. But "Mawdryn" is too explicit about its dates in a way that's too central to the story, so it amplified a previously minor inconsistency into an overt contradiction that could no longer be shrugged off.