This sounds like American colloquial, street-talk. It just means that she's thinks what they're doing is pretty bad. It could mean incredibly boring, or it could mean that it's just not what she wants to do. She's exaggerating the situation to be extremely awful, when it's likely just not her style. This originates from the phrasing of "this stinks" meaning that something is bad.

Grammar doesn't matter at all in conversation

Different places will tell you different meanings. It is simply a non-descript negative adjective, unrelated to stink, except that it means a negative as does "stink". . Here you can also call things "pretty stink" meaning about the same, just a general negative comment, without real meaning, except unattractive to be part of.