Only a very tiny part of the books is in MIX/MMIX (by my count from 3 years ago; details here: https://news.ycombinator.com/item?id=14520230) — if you don't care about it just ignore those programs/sections; it doesn't get in the way of anything. (Similarly you can ignore the mathematics if that's not what you want. You'd be skipping past a big chunk of the book in that case, though.)
On the other hand, MMIX is actually a very nice architecture for a programmer to write assembly/machine code for — it has lots of registers, etc. It's more pleasant to write in MMIXAL than x86 Assembly or (I imagine) ARM or even RISC-V. So if you just want that part of the programmer experience, it's a great way to have some understanding at all levels.
On the other hand, MMIX is actually a very nice architecture for a programmer to write assembly/machine code for — it has lots of registers, etc. It's more pleasant to write in MMIXAL than x86 Assembly or (I imagine) ARM or even RISC-V. So if you just want that part of the programmer experience, it's a great way to have some understanding at all levels.