For Dawn, losing Tara most likely felt like losing her mom all over again, I think aside from Willow, Dawn is the one that certainly loved Tara the most among the Scoobies, it's very evident that Tara was the person Dawn wanted to look up too, and she was the last person out there that truly took care of Dawn. Buffy did her very best but she was still absent, and the rest of the Scoobies never really got past the "Dawn is my best friend's little sister" phase. Tara was the best role model any kid could have, and she was the last of Dawn's family in the sense that after Joyce and Buffy died, and Hank left years ago, it was Tara that took care of Dawn, she became more than just a friend. So yes, Dawn lost a lot after Tara's death.
Does it compare, or surpass Willow's lost? I'm not sure, Willow lost her soulmate, it's pretty explicitly said that she was suppose to be her forever, so I'm not sure who lost more. Willow might have had a better support system than Dawn, but I'm not sure the two lost can be compared.
One thing I do believe is that once Dawn is older and mourned Tara, she'll have recovered much better than Willow did. Yes were shown that Willow moved on in s7, but I think she'll always feel like there's something missing. Bottom line is long term Dawn probably lost less than Willow, but in the moment, the lost might have been pretty equivalent.