It is useful esp for your target sets of schools.
As pp suggest "this is a large sample of a self selecting cohort" who will be the real contenders of the SW exams. Based on our experience it is a good indicator for the relevant grammar exam format results, to the extend that it is better than Atom learning SAS scores where it has little way to difficiate between the top set students - a lot of kids attend Sutton Mock will hit 130+ SAS in practice one way or the other, but you just can't tell if that is good enough - probably good enough for top private school until I heard a lot of london private take most 130+ CAT students in MN. 😏
From a slightly different angle, here is some elements Sutton mock might not be fully reflected the actual exam performance :
Exam format and weights:
Most of the superselective school will use written exam at some stage - comprehension, maths and CW. Sutton mock only marks MC English and Maths so your DD might want to spend time to practice those over the summer (or after September the first stage), in particular English and CW, the exam technique is vastly different, even the performance should be some what correlated.
Some schools use VR and NVR as test format with significant weights (not for SW London Grammars) they are very different type of questions, so less correlation to English or Maths performance. You might want to prepare for it if the schools you apply involve this.
Even for MC English paper from similar provider (for example GL) , different school exam will have different focus: some schools English paper will have no SPaG but all comprehension questions while some other schools test a lot of SPaG (for example sutton), so it is important to identify if your DD have any gaps in different area from the mock exam and improve.
Time:
There are a few months away from the actual exam. It is the time a lot of students start gaining preparation momentum and improvement. So be aware of your DDs progress and make sure it didn't plateau too early - consistent efforts, take regular short breaks, stay motivated will help.