The thing about Maribel is she isn't a typical HA so it's tricky comparing her to the others. Outside of Garret (and Deine but she's a Black anyways) the rest have a clear, heavy focus on defense while due to her stats and skill she's something of a bulky dps instead. As far as tanking goes, the fact that Maribel is often compared to Leanne, a Silver, instead of Gellius (a plat like her) or even Bernice (gold) says a lot about her full-time tanking ability. Also keeping in mind she'd have to tank an extra enemy if she AWs hurts her in comparisons to Leanne as well.

TLDR: If you have Bernice, Gellius or Deine and a bulky dps like Shizuka, Cornelia or Elva (there are other bulky dps, of course, these are just examples) then you don't really need to worry about getting a good Maribel. If your only tank is Leanne, or you have no bulky dps, she's a solid fill-in for those roles if you have no other options. Be careful if you AW her though.