Yeah, Memento is one of those cases where her stat screen doesn't tell the whole story--her attack speed is comically slow so she ends up bringing very little to the table in dps. Plus, it must be understood that even if she did witch level dps it still wouldn't be that impressive without the slow effect or heavy token use. Witches only get by because they're so cheap to deploy and the good ones bring a nice smattering of utility to the table--slows are nice on maps where you'd rather shoot the fatties than tank them, Yuyu can greatly ease wall of magic and Adele brings a global buff to the table. Just smacking things around like Cloris is a quick trip to the bench.
It depends on how you look at it/where you draw the line since with current content the role of off-tank is typically far less intense than that of primary tank. I often get away with using a CC50 Garrett in that role and many people understandably consider the poor dude's attribute distribution to be a dumpster fire. Basically, tanking is often about reaching certain thresholds in Aigis War and 3k health/450+ armor is about where units become sturdy enough to throw down with a pissed off Mummy King. Honestly, I think the biggest argument against considering Elva for a secondary tank role is that so much of her durability gain comes only after being Awakened. That's a big investment for a role that can often be just barely be filled by Leeanne. She's kind of in the same boat as Momiji--more viable/bulky than people often imply but not really a standout, either. At least her skill is better though.