Maribel is entirely magic/composite. Amniseri/Martena is the only thing that uses physicals, but that thing's also the one vulnerable to PAR and in any case the only thing threatening about Maribel is her last phase anyways.
MND based Yukari can comfortably slot 1 St Elmo's Fire, Suwako's magic phase, Reisen (physical using minion is easily PAR'd), Eientei (need a Star World Bracelet for Galaxy in a Pot), Evil Forge (no physical), Yuyuko (no physical), Yukari (no physical), Rinnosuke's elemental phases (only Huge Whirlwind and Sealing Slash), Rainbow Phoenix (lol), Great Intellect (no physical), Cosmic (no physical), Maribel (no physical), Bloody Papa (no physical), Hibachi's magic phase (only Ultimate Light Cannon), Celestial Bright Demon (Ultimate Light Cannon and Poison Thrust), Agastobrauma (Plasma Touch), Yuuka (Arrow Rain off 20k ATK), Eiki (Last Judgement but that's easy to work around), and Kedamagrammaton (no physical). It doesn't seem like a huge list, but it includes almost all the bosses that needs to be beaten (in a reasonable timeframe) to progress through the game, so I would still definitely say that she can see her share of the battle. (In fact, if there were anything I learned from the draft run I did, it was how pure MND tanks could actually be so relevant in slot 1. Iku now seems even more broken than I thought she would be before this!)
I do agree that DEF based Yukari could cover more threats more comfortably like Orin, Okuu or Master Light Wings, and be much more relevant for the 30F bosses, while still being pretty decent against what I've brought up. The thing is though, in Zoomy's proposed team Remilia and Keine exists, both of which handles physical and mixed tanking better (mixed tanking tends to require a lot more DEF than MND because of affinties being able to replace MND, so having good HP and DEF is better than having decent DEF and good MND), but there isn't anyone that's better at handling magic/composite based bosses than Yukari (Minoriko could do in a pinch but she'd have much more problem with composite attacks), so she's better off focusing on that area, and let Remilia/Keine handle the others.