I was reading about 雪莲。 It says “雪莲花香袭人，”
I'm pretty sure that ’香气袭人‘ is used to indicate a good smell, not a stench. Is that correct?
However, 袭 seems to mean 'attack, assail'.
In English, if a smell 'assailed my nostrils,' that would indicate a foul, nasty smell.
'Attack' or 'assail' also have negative connotations.
zdic has a meaning for 袭: (14) 盖 [cover], but that does not really mean a smell is good or bad.
Can ’香气袭人‘ ever indicate a bad smell? Why does a good smell '袭人’？
袭 in 袭人 is referring to 'to rush '
"袭人" means "rush upon people"
Example:
一阵阵恶臭袭人而來 - waves of horrible stench come rushing upon people
一阵阵花香袭人而來 - waves of floral fragrant come rushing upon people
強烈硫黃味袭人鼻腔 - a strong sulfur smell rush upon people's nasal cavities
花香 is a popular topic for poets and "花香袭人" (floral fragrant rush upon people)sound poetic but not any other smell. Phrases like "肉香袭人，酒气袭人，屍臭袭人" all sound unnatural because these smell are not popular topic for poets
Meanwhile, "扑鼻" (rush into nose) is as literary as "袭人" but less poetic sounding. "花香扑鼻，肉香扑鼻，酒气扑鼻，尸臭扑鼻" are all natural sounding literary phrases.
The word 香 means fragrant (when used as an adjective) or fragrance/perfume/spice (when used as a noun), so 香气 generally can only mean a pleasant smell by definition.
However, it is not uncommon in some context for it to be used to mean the opposite, such as when you are being sarcastic. So you have to be aware of the situation to interpret the correct meaning of the phrase.
The use of the phrase 袭人 is a literary device that helps to convey both the nature and feeling evoked by the smell as something that you can't see, but which overwhelms/covers you. Combine the two 香气袭人, and you create a very visual metaphor for describing a moment when you are suddenly overcome with a lovely scent.
According to the dictionary, the character 袭 has the meaning “触及” (to touch), as in “寒气～人” and “芳菲菲兮～予”.
