I didn't mind the anti-war message when the show was funny. "Sometimes You hear The Bullet" was a great episode, with a mixture of comedy and drama, and impressive acting by Alda and Stevenson. However, as the show progressed, yep: It got way too preachy, and what made it hard to watch was the show's taking itself way too seriously.
I really wish it stayed comical. Sure, I'd expect the characters to evolve, too. But they could have done that by making the show darker and perhaps more irreverentlike its movie counterpart.

The series became a bit too warm and fuzzy with warm & fuzzy Charles.
That's true.
The show went downhill BIG TIME.
It should've ended when Radar left.
The latter seasons' scripts, for the most part, were subpar or average.
There are a handful of ones that were comedic, but it's clear the show had seen its better days.
Then add Hawkeye's annoying and endless preaching. It really got old hearing his irrational anti-war rants episode after episode.