It is similar, but the tone of it is less surprised, and more sarcastic. A person might reflexively say, "Oh my god!" if they were genuinely shocked or surprised by something. A person saying, "Jesus tap dancing Christ!" is pretending to be surprised, usually to be funny or to be insulting, depending on the context of the situation. If a friend said this to me, it is probably because they are trying to make a joke. If a superior at work said this to me in a stern tone of voice, it would be understood as a rebuke or admonishment.

I would take it as equivalent to this trend (of softening harsh language): Oh my god --> Oh my gosh F*ck --> frick Jesus f*cking christ --> jesus tap dancing christ. Generally, instead of swearing, they made the phrase softer or funnier (depending on the situation). Its pretty common with english swear words. Depends on the person i guess. Personally, i would just say the harshest/most vulgar form unless i was around a teacher or someone who gets offended easily.

I have never heard anyone say that. I have heard people say things like "Jesus H. Christ," and you're right, it usually means "Oh my God" or something like it. Usually, though, it's a more emphatic statement and it's usually said in anger or frustration. *the dishwasher keeps breaking and leaking water in the kitchen* "Jesus H. Christ, why won't this stupid dishwasher work?"