Speaking personally, I would revise the heck out of this. I don't know if this is just a mockup for discussion purposes, something from a product, or what, but some of the changes I would make...
- The introductory two paragraphs of fiction? Cut those right out. Monster write-up is not the place for that, and it really doesn't add anything. You have the essential physical appearance in that 3rd paragraph (and somewhat in the 6th paragraph), which I would preserve (in italics) at the very beginning of the monster entry, just retooled to make it less a narrative & more a description.
- Way too much "often referred", "is regarded", "it is said", and "legend has it" in the Inspirational Lore. Not only is that bad practice with RPG writing in general, but it actually takes me as the reader OUT of getting inspired.
- I would heavily rewrite the entire Inspirational Lore section – in fact, I wouldn't even have a header for "inspirational lore", because the whole dang monster entry should serve that function. If a bit of writing is not inspiring the DM who will be running the monster... then what is the writing doing? If you need to break down the lore into bite-sized segments the bold italicized leaders that appear in the 5e MM are a fine way to do that.
- How do I use the Cailleach in my games? What are a few examples? If that's not evident from reading those bold italicized bits of lore, then it needs to be spelled out in bulletpoint form. For instance: The Cailleach's staff has been taken by the Queen of the Summer Fey, and eternal winter plagues the land; as the Cailleach emerges from her icy prison, the Queen of the Summer Fey sinks deeper into despondency.
- If attention is given to a Lair, I want more than (1) remote alpine ice palace, (2) wildlife spies, (3) helmed horror guards. First off, it doesn't feel fantastic enough to me – it's the sort of lair I could come up with pretty quickly on my own. If this were a product that I'd purchased, I'd be thinking, "huh, come on author, do some more of the heavy lifting for me!" I don't know your vision for the lair, so I can't for sure say what else should be included (given word count limits), but some possibilities: Who is Frozen in the Ice? (d6 table), What Horrible Future Do You Glimpse in the Mirror/Pool? (d6 table), Random Encounter Table, List of sights/sounds/smells, Map or Picture of the Lair.
- I do appreciate that you pointed out some of her minions (helmed horrors). Connections to other monsters / suggestions of typical monsters encountered with the monster are often left out in the 5e MM. So it's good you've at least included some mention here, even if just in passing. One thing I'm curious about is which 3 spells would these helmed horrors be immune to? This leads into my next point.
- Strategy/Tactics are not something that a DM should end up having to be experienced or visiting The Monsters Know website to at least get a sense of. Some sort of a nod to how the monster is intended to be played in combat would be appreciated. This is particularly true of more complex monsters with spellcasting.
- I'll second @Stalker0 's comment about long spellcasting lists being unwieldy in actual play – it requires a whole lot more prep from the DM to use such monsters effectively. But it doesn't have to be that way. Tighter more thematic spell selection, one-to-two-liner shorthand spell descriptions inline in the stat block, and eschewing spells (when possible/appropriate) for unique magical powers can go a long way towards making a monster easier to run. At least, IMHO.
- The "In Play" section felt really...barebones. Yeah, ok, she's ill-tempered and prone to flattery. That's a cliche, not a monster. You could drill down, for example: How does her ill-temper manifest exactly? Does she turn those who offend her into ice right away, collecting ice statues like a medusa? Or will she demand the tongue of the offender, which she magically places in one of her helmed horrors, granting it the ability to speak in the offender's voice? Now that's creepy and says something about the monster! Maybe you give some thought to ways to appease the offended Cailleach, setting up a sort of "role-playing" section with a very specific assumption about how the monster is used in play.
- The third tier of play – exploration – is something that I personally like to see explored more in monster entries. What are the signs of the Cailleach? I know you mention frost, ice, snow, animals spies in her Regional Effects, but I would go even more specific. Icicles holding distorted reflections of her victims hand from the eaves & trees in her territory. The tracks of elk suddenly turn into those of a four-toed humanoid – hinting that the Cailleach rides an elk and lost some toes to frostbite in her past. Stuff like that. Not necessarily for this entry, but more monstrous entries, some track & sign is a great thing to include if it makes sense & you have the space.
- Finally, I really miss inclusion of typical/example treasure. More broadly, making a list of rewards unique to the monster, which might include mentioning specific magic items in the DMG, unusual charms, themed trinkets, things you can harvest/craft from the monster, etc.
- EDIT: One last thing. The angled "margin/sticky note"? What's written leaves me cold (sorry for the pun). It's not really serving any function. Yeah, biting cold? For the Cailleach? OK, that's on brand, you didn't need to tell me that again. If you're going to call out text in such a special way, then that text needs to be special or exemplary in some form – e.g. a poem about the Cailleach, a riddle or two that the Cailleach likes to use, that sort of thing. There is a time and a place for such additions, but if it's not working, then definitely it's better to omit it from a monster entry.
Nothing wrong with being intentional and steering the monster's entry into specifics – the generic one-sized-fits-all approach you see in the 5e MM is a starting point, but IMHO it should not be a straight jacket. In other words: Consistency of stat block for ease-of-play? Yes. Consistency of overall monster entry just for consistency's sake? Nah, and it's probably constraining where the creative process could take you.
Hope that didn't come across too critical. I was trying to offer as precise feedback as I possibly could about why it wasn't working for me, in the hopes that my feedback would be actionable and helpful.
