I bought an SNES recently, and it came with a aftermarket power supply. Is this one at the correct amperage, etc.?
I just went and checked the power supply on my SNES (an original Nintendo power supply)--it's 10 V at 850 mA. Your supply is slightly lower voltage.
Speaking as an electrical engineer with some familiarity with how circuits were designed in the 80s and 90s, it's very probable that 9 V is enough. (current rating doesn't matter as long as it's high enough, by the way--the power supply could be rated to 10,000 A and it would work just fine. Though I'd question why you're using such expensive specialist lab equipment to power an SNES, if it was!)
Most of the SNES (and most 80s and 90s electronics) runs on 5 V, and there's an internal voltage regulator (most likely a linear regulator) that drops the input voltage down to 5 V. There may be some parts of it that run on 9 V (another standard voltage) as well, but that seems unlikely to me--the voltage regulators readily available at the time would typically require at least about 11 V to output a stable 9 V, and while ones that could drop 10 V to 9 V existed, they wouldn't have been as cheap as just using a higher voltage power supply.
It's also possible that some nominally 9 V parts are being run at 10 V straight from the supply; some things are perfectly happy on a pretty wide range of voltages. In that case, your 9 V supply would also be perfectly fine.
In any case, I wouldn't worry about it. Even if it turns out not to work, it's extremely rare for something to break from applying too low of a voltage. Too high a voltage can cause damage easily, but it takes some weird edge cases for too low a voltage to do any damage at all.
Do be aware, however, that low-quality power supplies can fail in ways that will damage things. If the power supply is working properly, everything's fine, but if it breaks, in the worst case you could end up passing mains voltage through to your SNES, which absolutely will fry just about everything on the board. Might be a good idea to replace it with one that's a little better quality; there are aftermarket ones out there that look a lot less sketchy than this one.
I'm assuming you're referring to the NTSC SNES released in 1991, due to the power supply being rated for 120V 60Hz, and due to asking about the SNES and not the Super NES Classic Edition (also known as SNES Mini).
Judging by an image found on an iFixit article for SNES outer case replacement (NTSC version) (click for larger version):
The NTSC SNES is rated for 10V at 850mA. Meanwhile, your power supply outputs 9V at 850mA. While it does have the correct amperage, it is short 1 volt.
Unfortunately, I can't answer whether or not this might cause any problems, and what these problems may be.