Interesting topic. And it's a bit tricky, isn't it? Anyway, I've just thought of two different meanings. But it's just me because this word seems to have a subtle nuance.
The speaker just wants to say 'it's not bad'. They seem to be implying that they are a little bit picky, and they honestly say they like it.
As good as other cakes
The speaker expected the cake to be dreadful, but it turned out to be as tasty as other cakes. 'It's a usual tasty cake.'
We Japanese often use that kind of phrase. In this case, "普通に" means not just that the cake tastes normal but also that he or she is not disappointed about that taste, I think.