I' having a knee replacement in 2 months. I just got a tattoo on my foot. Have I now messed up and delayed my surgery?

Likely to be fine ! A joint replacement surgery might be cancelled for any kind of skin infection or lesion on your leg. If the tattoo is healing without any issues, you will likely to be just fine in 2 months.
Depends, ask Surgeon. Some do not mind this due to distance from operative site and some time passage, others are very conservative. Of course they get more so when you have other illnesses (like diabetes or lupus, etc.), are older, and/or overweight. Call your surgeon's office now to facilitate rescheduling, if needed.