It's a matter of opinion. I disagree with your assessment of Kristophe's assessment of my assessment. Robin Williams has been well into decline for some time, and the ratings I gave were fair. I invite you, as I did Kristophe to post your own ratings list, so that we can better get a sense of your grading style.

The movies was full of transparent plot devices designed to elicit an emotional response, much in the same way that Brokeback Mountain is meant to be "controversial", or that North Country is meant to be a "break-through" (if you want modern examples). Weakened by the "inspired by a true story" parameters, it's a predictable plotline, and Williams himself is reduced to making pathetic speech-ettes about happiness and the disease state. His comedy bits were good, his dramatic acting shamefully poor.
One of the worst movies of all-time was Martin Short in 1994's "Clifford", which was a similar theme. These aren't enjoyable movies for audiences older than 6. Generally, movies where the lead character becomes a small child, an animal, or switches bodies with a member of the opposite sex, are some of the most terrible atrocities inflicted upon an audience. I don't even need to see the new Tim Allen movies where he "becomes a dog, then back to a man, but with doggie-style" to know that it stinks.
It's a special-effects blitz, not a serious kid's adventure movie. The thing is devoid of plot, and Williams' only job is to look grizzled and direct the kids to scream. Williams starts to rant, and display something at times, but it's surely not humor.
Tootsie wasn't great either, but it also wasn't full of slapstick. The movie is replete with Mary Poppins-isms, but always with the hidden identity of the dad lamenting the "loss" of his kids. The film becomes about Williams, since there is no plot, other than the disguise- there's no danger to the kids, no goal in sight for the poor father, except to eventually be unmasked and get a biiiig hug from his crying kids. Those aren't character flaws- it's predictable tripe. Guys in drag are funny though...
The movie itself has very few redeeming points. The sequence on the Moon is at best, bizarre and acid-tripped. At worst it's unscripted nonsense designed to fill in a few moments of what is otherwise a very bland, and terrible kid's film. I don't need to give a more thorough review of the movie, as more literary a critic than I have ripped the film a new one, to no end. But I will say that Williams should've played the Baron, and not the awful Moon-man, a sequence obviously longer on make-up and special effects, than writing or performance.

Yes, but even I will submit that Buckaroo Banzai is terrible in places. It is too long, too full of comic book logic for its time (a pre-Daredevil or pre-Batman), and suffers too much Jeff Goldblum (is there a movie he doesn't look out-of-place in; no, not "The Tall Guy", either ). But there's no way I can diminish the zany characters, the great lines, and the amazing virtuoso performance of John Lithgow. That's why BB isn't bad, it's "cult", and a true one too, unlike Baron Munchausen.