Yes and no.
I mean, the Mac is a cut-down Lisa. But you're right, cut down too far.
(Aside: the Lisa was a 68000 as well, I am fairly sure. No MMU, but it did have multitasking.)
I think the problems were in scaling it down far enough to get the price down to vaguely reasonable.
That meant:
• 128kB RAM (same as the Sinclair QL, which launched 2 weeks earlier)
• No hard disk. Keeping a full multitasking OS when running from 400kB single-sided double-density floppies -- indeed, from one floppy -- would have been significantly hard.
Before an outraged Amiga fan jumps in: yes, I know Amiga did it, but that was 2 years later, with 4x as much RAM and floppies holding 2x as much data. Tech was moving fast in the 1980s.
A 512kB original Mac with 800kB floppies was technologically doable in 1984, I suspect, but instead of costing $2500 when it launched (~1/4 of a Lisa), it would have been $5000 or something, and I strongly suspect it would have flopped too, as a result.
My key point is around the operating system. They had a system in LisaOS that had multitasking, memory protection, proper dynamic allocation, etc. It was a better OS than what was shipped on the Mac. It was unfortunate they threw it away. Likely due to personal & organizational politics more than technical and cost reasons (Jobs hated the Lisa). A few years later they were running Mac OS on old Lisa hardware.
Put it this way: the Lisa had an MMU, memory protection, and an OS with better multitasking. Yes it was expensive to make, but they could have scaled it down instead of building a whole other, competing system that was technically inferior from an operating system perspective.
The original Mac was built as if it was a one-off product, and they lived with the results of that for two decades. And these were Jobs' choices.