In Thor: Ragnarok, we learn that the eternal flame of Asgard could re-animate people then why wouldn't they use it to bring back people close to Thor? Does the eternal flame reanimate only evil people?
During Thor : Ragnarok, the Eternal Flame is explained to be the true power of Surtur, along with his crown. When you combine Surtur's crown with the Eternal Flame, you are able to resurrect Surtur and bring out his true form, which is able to destroy Asgard singlehandedly.
Conversely, we've seen Hela resurrect Fenrir and her army through use of the Eternal Flame... albeit it took on a distinctly green color, which is associated with Hela, never with Surtur. Here's my guesses :
- Only Surtur can fully resurrect through the Eternal Flame. Hela was only able to twist the artefact through her own magic so she could summon her army from the dead. They all have a limited, animalistic level of conscience with absolute loyalty to Hela. I don't believe Fenrir being seemingly "intelligent" is a plot-hole, as he's roughly as smart as the average zombie soldier from what we've seen.
- The Eternal Flame can resurrect any and all dead creatures, though it only gives them a limited amount of intelligence. Basically the same as before, but anyone can wield it.
Personally, I believe guess number 1 to be true. We've seen Loki's illusion and various magic spells to often devolve into or stem from green arcanic energy, which when combined with Hela's own emerald color, would mean Asgardian magic (shared by both Hela and Loki, although on differing scales) is simply green.
Now Hela is an uber-powerful warlock on par with Odin, which indicates a very high level of power is needed to trifle with the Eternal Flame. Hela openly dissing the Space Stone also confirms the Eternal Flame's power is far higher. Given that Odin is the only other individual who could have wielded that flame, and given he is an irrational racist certainly unwilling to manipulate Surtur's magic + the Eternal Flame does not seem to grant sentience, it's pretty obvious why they never used it on Thor's mother.
They didn't want a zombie grandma lying around.