It looks like you’re engraving the front of the glass(?) mirror. Cleaning solution shouldnt cause too much of an issue, but you never know.
If you’re engraving onto glass and seeing that sort of variation… I’d suspect the art, like @marmak3261 mentioned. If you upload the art file we can take a look, or if you know your way around curves, i’d take a look at the histogram and see if you have any weird dips or peaks in the colors, which might indicate a variation in what you’re seeing as “black”. (You are working in a black and white raster image, yes?)
You also don’t say in your settings what mode you’re using. 3d engrave (aka vary power) or dots or what.
Anyway, I’m already way into speculation here, you need to provide a little more info for us to really be of any help now.