"Big Yikes" is a more intense version of the word "yikes." It refers to something that is so very embarrassing that another, much larger "yikes" is needed.
Brody: Yo, did you see her outfit today?
Sarah: Yeah, it didn't even match
Tod: Big yikes!
A reaction to something that is normally bad.
post "Mulan actor in tweet says they support Hong Kong police"
person" geez that's a big yikes"