I'm inclined to agree to some extent. I'd have preferred MM to have been left unwritten until the plot made more sense. There are some good ideas in it but on the whole it's difficult to see, by about half-way through, how the various plot threads COULD be brought to a conclusion.
I like the whole Mr Bent / Miss Drapes sub-plot but some aspects of it, such as the hanging sheds in the caves under the Bank, don't work for me. Gladys the love-lorn golem is a good joke, as is Mr Fusspot's new toy, but Professor Flead could have been omitted entirely since Moist appears to solve the problem of controlling the golems without his assistance and he doesn't really tell Adora Belle Dearheart anything useful, either.
The protracted and elaborate Cosimo/Vetinari sub-plot isn't worth it for the gag about the "Vetinari" ward at the asylum.
Ultimately, all the DW books are joke-driven. If the basic joke or premise is good enough, the whole book works but if not, and in MM it isn't, then piling more secondary gags in won't rescue it. This has always been the reason that Carpe Jugulem has always been my least favourite book; the basic joke isn't strong enough to sustain a whole book. The Last Hero has a touch of this, but is saved by its brevity and the illustrations. Thief of Time has always struck me as having too many, and un-necessarily complex sub-plots ( especially the Yeti ) culminating in a notably contrived deus ex machina ending
I'm also unclear as to quite what DOES happen regarding Igor, Hubert and the Glooper. There seems to be a continuity error regarding the situation regarding the gold, or possibly the different sub-plots require inconsistencies which have been left in as the plot requires. There are various continuity errors or simply changes from book to book, usually driven by the requirement of the plot of the moment, and so what? But in one book, not so good.