Popette

Depends on the context!!

Very different:

I love lemon tart. It's sweet but not too tart

I really don't like her she is a tart

If you mean the first one in Spain we say tarta or pastel, and in southamerica I think they say torta, but don't quote me on this, best to ask a native. The second one I think is ácido (like lemon) or agrio. The third one..depends also how harsh you want to be, something like fulana??