Haeckel made a conjecture based on his observations. He may or may not have slanted his drawings to support his conjecture, but there is little about his drawings that a non-biologist would classify as fraudulent. I would guess that a layman presented with his drawings and actual photographs of embryos would not be able to identify the significant differences.
His conjecture was that embryos resemble the adult forms of their ancestors. This is not true. A more recent conjecture, also partly wrong, is that embryos resemble the embryos of one's ancestor species. The full truth is more complex.
There are lots of examples of conjectures being wrong. The early Lowell drawings of Mars are inaccurate. There is no doubt they were distorted by an imagination that wished to validate a conjecture about Mars. But they were not fraudulent. And they generally appear in textbooks as illustrations of historical events in science. They are not presented to illustrate proof of Martian canals, and Haeckel's drawings are not presented as proof of his obsolete conjecture.