To answer your question directly,
Frank Miller
Ian Bertram
Flaviano Armentaro can have some rough compositions at times.
Rob Liefeld remains a strong go-to.
Squirrel Girl is meant to evoke a small press or indie comic style. Like so:
Wired Up Wrong
Rock Candy Mountain
Anything Raina Telgemeier (currently one of the biggest graphical novelists)
The Best We Could Do by Thi Bui
Again, this is largely a specific choice, which is why they hired Ryan North as writer. North previously did Dinosaur Comics and Whispered Apologies. Examples of both:
Occasionally, the Big Two like doing things like this. Why? Because while these things tend to not sell in the direct market, they sometime do better in traditional book markets. See the difference between
Dec 2017 in the direct market and
December 2017 on Bookscan.
#1 in direct market:
#1 in Bookscan
The reason Marvel keeps Moon Girl and Squirrel Girl around is they reach different audiences.
Now as to the overall thrust, you seem to prefer Japanese works and artists, hence the references to Guruhiru and Sana Takeda. Which is fine, you have an aesthetic preference. Henderson's art isn't my cup of tea either. At the same time, I like you do the art of Henderson a horrible disservice. It's a similar issue I tend to find when I offer One Piece or Attack on Titan as an recommendation for folks.
Or even moreso, Mob Psycho 100, the original One Punch Man, or Pop Team Epic:
"But Mike, those are weird comedy and gag manga!"
Yeah, that's the point. Squirrel Girl is aiming for a style and tone. Henderson is largely suited for that style and tone, which is why the book works for those whom it works for. Which makes the thread pretty much a big "I don't like a thing and would like another thing," which is fine but not really novel in thesis or scope. My answer is "So?" and "Comics do different things."