Spotty - (N. Amer.) of uneven quality; patchy
uneven - not uniform or equal
What is the meaning of quality in this context?
“quality” here means degree of good/bad.
A common example would be wireless signal as you move around a city. In some places (or spots) it’s good, and in others it is bad. We could call this uneven coverage “spotty”.
That sense of "quality" means "how good things are". If they are defective, they are of low quality. If they are flawless, they are of high quality. If quality is spotty or uneven, some examples will be good, and some will not be as good.
American Heritage Dictionary "quality"
3b. b. Degree or grade of excellence: yard goods of low quality.