Yes, creepy. It is creepy for me to eat in a place where I am the only one (or one a few tables) dining.

Decently filled, to me, would be at least 10%.

Size does not matter, although I suppose at a smaller establishment this would be less of an issue, such as in a 10 table restaurant.

This applies at all price levels, in fact, the nicer the place and more expensive, I much prefer there to be other tables occupied. For example, I had lunch at a fairly upscale steakhouse in NYC a couple weeks ago at around 1pm. This is a pretty popular place and there have been times that I cannot get a reservation there for dinner. There were 3 other tables dining and probably 30+ empty. I just don't like the feeling of an empty restaurant.

Time could play into this, if one goes to dinner at 3pm, they should expect an empty restaurant. If one goes to dinner at 7pm or 8pm, this should not be the case.