Yes, people thought low fat diets were good but your body needs good fats such as polyunsatuarated.
You need good fats, not trans or saturated fats.
Trans-fats are totally different from saturated fats. They should not be lumped together in any way.
Saturated fats (as found in natural animal products) don't contain trans-fats.
It's the man-made stuff that contains trans-fats: margarine, vegetable shortening (Crisco), etc... You know, those "healthy alternatives " that for decades the government was telling us was so much better for us than animal fats. Excuse my French, what they told us was a pile of BS.
A few interesting things when it comes to trans fats.
1) The recommended safe level of trans fat consumption is zero. In other words, trans fats aren't considered safe by the medical community at any level.
2) If there's less than .5 grams of trans fat in a serving, they don't have to list it on the label. So, even if it says 0 trans fats on the label, it might still contain some. You could eat four servings of say a 0 trans fat potato chips, and still consume almost 2 grams of trans fat.
3) Mary Enig. It's worth looking up a bio on her. It's fair to say if it had not been for her persistence in the 1970s-1990s the dangers of trans fats may never would have come to light. She was something of a medial whistle blower, and suffered the consequences. She was threatened at work (U of Maryland); her department was downsized and virtually eliminated; she was blacklisted from medical journals; and she was ridiculed in public for pointing out the dangers of trans fats (and for pointing out that the industry had know about the dangers of trans fats since the 1950s). The wikipedia article on her doesn't come close to describing the **** she had to put up with.
Enig's story is also a good lesson in how industrial interests shape government policy and recommendations about diet and health.