I was going to answer this before Phil did, but was interrupted. However, since he hasn't said what I was going to, I'll add my thoughts.

"Nice" is easy: it definitely means tasty, delicious, appetising. The trickier word here is "naughty".

There was an TV advertising campaign for cakes in the UK in the 1970s and 80s, with the slogan "Naughty, but nice". Supposedly the slogan was devised by the novelist Salman Rushdie, working as a copywriter, though actually the phrase goes back at least as far as a 1939 film starring Dick Powell and Ann Sheridan. If this text is British, I think there's a strong chance the writer was thinking of these adverts and expects some of his/her readers to remember them, as I do.

Naughty, as Phil has said, means wrong, but only mildly so, and above all it implies pleasurable wrongdoing, the pleasure of transgression. In relation to cakes or muffins, "naughty" really means "fattening". Also unhealthy, because of cholesterol, heart disease and all that, but above all departing from virtuous eating. So muffins are fattening and unhealthy because they're full of fats and carbohydrates.

However, this is supposed to be a healthy muffin, so I don't think they're using "naughty" to mean unhealthy, or even fattening, because that would contradict their message. I think the naughtiness here is mainly the pleasure of eating something you know you really shouldn't but that is very tempting, very nice, but in this case having the pleasure of giving yourself a treat without paying the full price in weight gain etc. "Naughty" can't actually mean "healthy", but it doesn't really mean "unhealthy" either. I would say it means tempting and self-indulgent, because eating muffins, even healthy ones, is a guilty pleasure, a departure from the path of dietary virtue.

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Note added at 53 mins (2016-03-10 06:52:59 GMT)
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I meant to put "enticingLY self-indulgent" in the answer box.

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Note added at 1 hr (2016-03-10 07:04:18 GMT)
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I think I've sent a somewhat confused message here. I would say that "naughty" in this context, even in the "Naughty but nice" cake ads, only means fattening or unhealthy indirectly. It mainly means pleasurely transgressive, the mildly guilty pleasure of doing something you know you really shouldn't.

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Note added at 1 hr (2016-03-10 07:18:43 GMT)
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That's how I see it too. Virtuous eating is a typically Anglo-Saxon protestant concept, I think: not just a means to the end of being thin and healthy, but something good in itself — virtue as its own reward. It's often said that the dullness of traditional English cooking is symptomatic of a culture that regards sensory pleasure as morally suspect. "Naughty" is a word mainly applied to children, and being naughty is fun simply for the fact of breaking the rules, in a culture that instils stifling conformity as an end in itself.