She had three things working against her:
1 she married the widower of a very beloved Queen Astrid who died way too young
2 When Leopold III married her he was a prisoner of war being allowed to live at Laecken Castle while other Belgian servicemen were in camps away from their families. As Lilian was pregnant he did not have the option of waiting a few years.
3 she was a commoner marrying into a reigning dynasty. Her brother had only a few years earlier denied his brother Charles, count of Flanders to marry the woman he loved because she was a commoner.

As a result Lilian was always seen as a gold digger, someone who slept her way into the Royal Family and took a place that did not belong to her. In fact she did make her husband happy and gave his children from the first marriage a much nicer childhood than they would have had otherwise. I think she may have been a warmer/nicer stepmother than she was as a mother to her own kids. For them she was quite ambitious and rather strict.