Beginner's LTSpice Question: Why does my sine wave look so bad and how can I fix it?

That voltage is rather low.

Simulate->Control Panel and set Absolute Voltage tolerance to something smaller like 1E-7 or 1E-8.

You could use @Spehro Pefhany's answer, but there is another way to do it: since you only need a fixed amplitude sine, don't use a behavioural source, instead, use a simple voltage source with SINE(0 {disp} {f}). This is because behavioural sources are a bit more tricky and need tinkering. If the displayed waveform is still not satisfactory, then disable the waveform compression. By default, all the points are compressed leading to artifacts. You can disable this by adding .opt plotwinsize=0 to the schematic. This setting should be used whenever you need details. The prce to pay is larger .raw files (no compression). Personally, I'd recommend these, first, and only if you have problems try altering the settings in the Control Panel > SPICE tab.