"By the end, he barely even APPEARED. It was almost exclusively the Morgan and Edwina show, and since I utterly despised both of those characters ..."
Plas appeared more than Morgan and Edwina. It might seem like he appeared less to you if you hate Morgan and Edwina.
"Which was easily one of my least favorite issues, since I would have found it dull, unimaginative and repetitive even as a grade school-aged child."
I found it fun, imaginative, bizarre and funny.
"While I admit that I enjoyed some of the pot-shots against then-current DC continuity (as well as against Bush, via Woozy impersonating Luthor), in large part, they were endemic of the problems with the book as a whole, in that it had no idea who it was for, or what it wanted to do."
It was for the readers, the book wasn't just aimed at kids, and it wanted to make us laugh.
"It was a book in which the title character made way for a stereotypical "strong female character" so utterly one-note and anti-charismatic that she might as well have been Mark Waid's Sharon Carter (and as a bonus, Morgan was completely rewritten to not be the villain of the first story in which she appeared, literally at the last minute, making her even more of a poorly conceived cipher), plus a sulky and equally stereotypical goth girl, who appears to have been drawn from all of five seconds of research, consisting of glancing at the Hot Topic site."
And Woozy is the stereotypical fat dumb guy. These characters are intended to be funny, not complicated, complex and dramatic.
"It was a book that saw fit to devote an entire issue to a "Plastic Man versus a mouse" plot so inane that even the average young Tom & Jerry viewer would have found it plodding and weakly-written by comparison,"
I found it more entertaining by comparison. Jerry never actually dies in the end of a Tom & Jerry cartoon. I love the Plastic Man versus the mouse issue because it's darkly funny and bizarre. The plot is refreshingly simple and amusing, full of visual humor.
"but then followed it up with issues which hinged entirely on incredibly obscure points of DC continuity (to the point that I found Baker's commentaries on DC self-indulgent EVEN WHEN I AGREED WITH THEM, which I mostly did), plus issues in which rape and child abuse came to the fore (the one-liner about Dr. Light and rape - "It's like it's his new power now" - was grimly accurate, but did not belong in the Plastic Man book, and the scene where Morgan nearly BEATS Edwina made me go WTF, especially since it was in the context of both characters already being portrayed as utterly unlikable in just about every way)."
Darker humor is fine for Plastic Man as well as the more light-hearted humor. A man attempts to commit suicide in Jack Cole's Plastic Man series, the Granite Lady story.
"As such, artwork aside, the book failed on pretty much every single front that it's possible to fail"
It didn't fail to entertain me.
" - even as a kid, I would have been insulted by how dumbed-down and mono-dimensional its characterizations and plots were"
I'm less analytical about humor and I don't take humor seriously. This is a comedy, not Watchmen. You could call The Three Stooges dumbed-down and mono-dimensional, and their funny to me.
"and even as an adult, I felt skeeved out by scenes like seeing Morgan nearly SLAP her own adoptive daughter."
That made me laugh. Don't take it seriously. Edwina was a brat and I hate political correctness in a comic book.