Everyone generally will take the same route yes, it mainly just depends on how well you take these routes, which depends on how good your "lines" in this game are; or basically how fast you go through the course. The best line for any course is the one which will give you the shortest lap time possible. Someone who has practiced a course and has really good lines racing against someone who has mediocre lines will almost always result in them winning the race, clearly. Getting **** on by a hoard of blue shells or whatever can affect this outcome, you are right; but 49 out of 50 times, the good-lines person will win. You will definitely have to factor in the player's knowledge of various shortcuts, all ranging far and wide in terms of difficulty, as well. A prime example being the double on DK Summit, or the lava cut on Grumble Volcano. Someone who doesn't perform shortcuts or doesn't perform them well will have a very hard time against someone who does. However, ultra-shortcuts should not be factored into someone's "skill"; and you are an extreme noob who I have lost all respect for if you do them.
Other than lines and knowledge of shortcuts, a person's item-usage has a definite large chunk in terms of their skill as well. Generally in your typical worldwide, you will get a lot of people who have no idea how or when to use their items, and they will generally just do whatever random thing they can come up with, which usually results in spamming good items almost immediately, such as the Shock. While this can be annoying at times, it also works to an experienced player's advantage in terms of predicting when shocks will happen. The shock doesn't appear until ~30 seconds into the race, if it or a blue shell appears before then, you are racing with a hacker. Typically, as I said before, when the average player sees that shiny Shock in their possession, they are for sure going to spam it right away. This is your opportunity to dodge it with simply just knowing the location of all the item boxes on the minimap, and paying attention to when the bottom 2 positions pass over the first item-box set after the 30 second mark. If you have a power item (Star, Bill, Mega) and you see those bottom spots pass over the item boxes after 30 seconds and you use your item, I guarantee you will have an incredibly higher chance at dodging the shock. This same prediction-rule applies to more experienced players who like to "target shock" others as well. If it has been passed 30 seconds, you are entering lap 3, no shock, and 1st place has somewhat of a small breakaway and are approaching the jump to enter the Bowser-room on Bowser Castle Wii... It would probably be wise to use your star or whatever just before they hit the ramp. This is an extremely popular TS-area, and I would not be surprised if a shock were to happen and you were to dodge at all. Predicting and dodging the Shock is key.
BACKING away from the Shock though, because I could go on about that item forever, creating traps with items such as bananas and FIB's is very useful as well. Generally what I mean by this is placing these items right after corners which are generally taken very tight, or on long sprial-turns such as the ending to Bowser Castle N64. Due to the tightness of these turns, it can be generally easy to get people with whatever item you leave for them to hit. Simply because they do not see the item until it is too late for them to be able to react and dodge it. This doesn't only work with stationary items either; and is also possible and potentially easier with backspamming shells as well. However, backspam and traps such as the ones listed above are very easy to predict and thus very easy to dodge at times. You may also get players who place these items slightly wider in anticipation of you going wide to dodge.
ANYWAYS, I could go on about other things like target POW's and blocking certain shortcuts, but I've typed 87x more than I wanted to in the first place xD. Basically, there is skill to this game, and it does vary quite a bit simply on how well the person's lines are, how knowledgeable they are about shortcuts, and how well they know item-usage.