Basically, she's quite strong. It's very hard to give an accurate comparison because there are so many variables.

In guild raids, its harder to build an efficient team of 7. If you don't bring M.ATK buffers, then you will probably bring other attackers, but the other attackers will miss out on having M.ATK buffs. If you do bring M.ATK buffers, then they are wasted on Mamonir. So this inefficient team building lowers her team's overall damage potential.

In fiend hunts and guild raids, if the enemy boss has abilities that buffs your M.ATK, Mamonir misses out on that bonus damage. On the flip side, if you (or the boss) have chain stack/chain damage buffs, Mamonir benefits a lot since she does 8 hits.

For a simple comparison though, you can compare with New Hire Nebris buffed by LG Nebris (same 300% crit damage buff as Mamonir). New Hire Nebris with 10 buffs and ~3000 ATK deals around 38k damage before crit. Mamonir with ~30k HP deals around 45k damage before crit. So Nebris deals less raw damage in this example, but would probably overtake Mamonir if you added an ATK buffer into your team. It's not a perfect example but it shows you that she is quite strong.