The user manual file you have linked is in zip format so I didn't download it. Looking at pictures on the web it seems that the instrument is a large multimeter style device.
Figure 1. One of the Hantek range of instruments.
I notice that this device has a USB port. The big danger of this is that there may be no isolation between the USB ground and the COM pin on the scope. If there is no isolation then connecting the COM pin to a live circuit while the USB is plugged in may cause high current to flow through the meter's ground destroying it and the USB source. The outline specification does not mention this. With USB unplugged there may also be a shock hazard on the USB port. Check this very carefully and understand the limitations of the device before use on mains circuits.
Can I simply connect the probe tip to the outlet on the dimmer channel I want to measure like on this image (not my image) and the ground tip to the metal chassis for grounding?
Normally you would measure between the line and neutral, not ground as there may be a slight difference between the two.
The 10x probe setting reduces the voltage seen by the 'scope and 230 V is within the 300 V RMS specified. There isn't a lot of safety margin there and at the price these are sold at I'd be concerned that safety might not be quite at the top of the priorities list.