The one thing I really don't like about Yukari as a tank is her HP. It's bad. Really bad. Potentially worse than Tenshi bad, given how much more expensive it is for Yukari to raise it with skill points. In my latest playthrough, Cirno consistantly had more HP than Yukari, and Cirno only has an HP growth of 11, which isn't good to begin with.
What I would consider doing instead is focusing on just MND for Yukari, and make her into a magic tank. Magic spells tend to have less piercing formulas than physical attacks, so it's easier to nullify all damage after buffs and make Yukari take 0, at which point her HP doesn't matter. For physically-based bosses, raise up Youmu as your main tank. Youmu has much better HP and DEF than Yukari, and could be made into a decent physical tank for specific bosses without too much loss in her offensive capabilities. And since she's your tank, it's easy enough to just leave her out to take advantage of her unique Focus skill.
The vast majority of bosses in the game tend to focus on just hitting either DEF or MND, making dedicated single-stat tanking a viable strategy. Observe: (SPOILERS ALERT!)
Youmu - DEF-based. Myon has a few magic spells, but they're not that strong or frequent.
Alice - Mostly DEF-based. Beautiful Light's attacks are stronger than Magical Light's, and Alice has 3 DEF-targetting attacks and only 1 MND-targetting spell (Seeker Dolls). Dolls War, Alice's strongest attack, hits DEF, while all single-target attacks (which are what you want your tank to handle the best) are DEF-based as well.
Yuugi - Completely DEF-based, duh.
Nitori - Completely DEF-based.
Reisen - Completely MND-based. Her melee underlings shouldn't be able to get a single attack off if you know what you're doing.
Tenshi - Completely DEF-based.
Eientei - Almost completely MND-based. Eirin's Shadowstitch and Galaxy in a Pot are the sole exceptions.
Yuyuko - Completely MND-based.
Orin - Mixed, but DEF is more important, due to Cat's Walk spam.
Mokou - Believe it or not, almost completely DEF-based. Only Flame Wind hits just MND; everything else is mostly physical (including Fujiyama Volcano), with Tsuki no Iwakasa's Curse as composite.
Flandre - Focuses more on DEF. She has 2 spells that hit MND, but both are Fire-elemental, so slapping some Fire affinity onto a physical tank might be enough. Everything else is physical or composite.
16F - Completely MND-based.
18F - Start with DEF, then switch to MND after the 1st form. Watch out for Sealing Slash, Huge Whirlwind, and Explosive Flaming Sword.
Final Boss - Completely MND, if you can keep the Shredding Amnesieri paralyzed.
As you can see, almost every major boss can be conquered by focusing on tanking with just a single stat, which favors using specialized DEF and MND tanks. Interestingly enough, there are more DEF-based than MND-based bosses, which is further reinforced by the fact that many magic-using bosses can be overcome with affinity instead of high MND.
