Tahini dressing too lemony. How can I fix it?

I would simply drizzle in more tahini until it's back in the flavor range you prefer, then adjust the salt and liquid (water and/or oil) to your desired consistency. You can also tone down acidity by neutralizing with a pinch of baking soda (go slow! it's easy to overshoot) but this won't reduce the non-acid-based lemon flavor.
Before you adjust it, do try it with whatever you've planned to dress with it (if you haven't already); I often find that tahini dressing has to taste very lemony on its own to taste right in an assembled dish.