Percentage lifetime used on my SSD: 90%. Is that good or bad?

It means your drive has 90% "life" left.

After some additional research, I came across this Crucial.com article that mentions the following:

In regards to the attribute named “Percentage Lifetime Used” (sometimes referred to as “Percent Lifetime Remaining”), this is simply a metric for how much wear life is left on your SSD. A solid state drive, like any flash memory-based storage device, has a limited amount of data which can be written to the memory blocks before they start to lose their reliability, and eventually go into read only mode. Your Crucial SSD will keep track of this life with SMART attribute 173, “Average Block Erase Count.” The Lifetime Used is a reflection of the block erase count in terms of a percentage. For example if your drive is rated for 3000 block erases and you have a total of 100, your Percentage Lifetime Used would be 100/3000, or 3-4%. For percent lifetime remaining we would simply take (3000-100)/3000 = 96-97%. These attributes are not a full picture of the health of a drive, but an expectation of how much usable life is left.

Because the attribute is vendor-defined, it can be complicated and confusing to figure out how each drive implements it.

Try running the Crucial Storage Executive tool to get a more accurate SMART reading that (should) follow Crucial's definitions by the book.