On the moral and narrative side Truth is a winner.
On the pragmatic side I think it is likely that Mezmerella is going to hypnotize Mirage or something if she suspects that Mirage is downplaying her involvement. If we are going to lie then it can't just be "I didn't do enough to stop him". It has to be something sufficiently damning that Evelyn could hold it over Mirage's head even if it isn't as bad as the truth.
Remember the truth is that Mirage recruited former superheros under false presences so her boss could 'accidentally' murder them with his death robot. That makes her an accomplice to serial homicide and/or genocide. Judging by the scene where Mr Incredible is looking at the Kronos computer over 18 people were killed in the Omnidroid tests. There is a lot of blood on Mirage's hands.
[x] Tell them everything
The jig is up, your own people have clearly begun to mistrust you, and you owe Hego this much. You… you don't know if you can bear this weight anymore. You will give yourself, at least, to a man whose judgement you trust.
Reasons being twofold, the first is that yeah it really is fitting for Mirage's story for it to all come to a head here. The second is I think there is a real possibility that Mirage is lost to both us and herself if we were to lie. Maybe not physically, but she would be consumed and defined by that choice forever and that's a bad end for her.