Crisis on Infinite Earths affected the entire multiverse... billions and billions of lives were extinguished, and the story included hundreds of characters in only twelve issues. With this in mind, we can say that Mera's participation was somewhat significant in the saga. She was part of the team of super heroes who went to Earth-S to fight the supervillains that froze it over. Mera and Aquaman fought the Ocean Master, the Icicle, and Captain Cold.
Mera was also in Atlantis when Aqualad brough a very badly hurt Tula A.K.A. Aquagirl, who had been attacked by Chemo in Earth-4. Unfortunately Tula died and Mera was there to deliver the bad news and console a broken Aqualad.
When all dimensions were consolidated into a single one, and the final battle started, Mera and her allies were defending the capital city of Poseidonis from the Antimonitor's killer shadows. It was here that Mera witnessed the horrifying death of Lori Lemaris, the mermaid who had been at one time Superman's girlfriend.