“Oh my gosh!”
– grammatically ok
“Like, who likes to eat cheese, like, that’s so yesterday, like, you know, duh!”
– several issues for strict grammatical correctness, unless you accept Valley Girl as the norm.
* “Like” is just an invented almost-meaningless interjection, so not really grammatically incorrect, just dialect.
* “who likes to eat cheese” – content is ridiculous but grammatically should be punctuated with a question mark and a new sentence, not a comma.
* “that’s so yesterday” – should end with a period. Changing meanings of words is slang/dialect rather than grammatically incorrect, but it also annoys people with more developed language skills because it is so minimal in its meanings and so high in its assumptions and the speaker’s assertion of rightness about something so vapid.
* “you know, duh!” – again slang and nearly-meaningless assertion combined with implied assertion that if you don’t agree with the near-meaningless assertion, you are the mentally deficient one. But not strictly grammatically incorrect (except “duh!” was recent-ish slang in 1980–84).
“Like, seriously!”
– grammatically sort of ok
As for what I’d think if a foreigner were talking Valley Girl? I’d probably think it was a hilarious phenomenon, and (intentionally or not) a well-deserved ironic commentary on American culture and its spread. It seems to me that Valley Girl works best when being ironic and sarcastic, and can be funny in moderation, but that it also has a hard time communicating more than reactions to things and sometimes passive aggression, and a way to avoid actually saying anything with direct content in it. It can easily annoy people and provoke negative opinions.
So, as a joke in moderation, it may be very funny.
As a way of being all the time, it’s slightly horrifying.