In Mario Kari Wii (which I played way too competitively back in 2010) the blue shell is not at all an issue if you play to win. If the other players fail to catch on, its existence can only help.

First, learn all the shortcuts in the game. Some of them are quite good and they're all online. Then:

For all but the last lap your job is to stop anyone from getting far ahead and to lay back in about 8th place. Cycle through boxes (just use them) until you get a star, ideally. Hold on to that as you enter the final lap.

Half-ish way through the final lap, while still in a far back position (5th to 8th), use the star right before you pick up your next box. Since you picked it up while in a poor position, it will be something good like another star or a golden mushroom.

Thanks to the rubber-banding (such as the blue shell!), first place won't be very far ahead, and the star should advance you to near first, if not first.

When a blue shell comes at you, you will have the ability to dodge it with the mushroom or invalidate it with the star in your inventory. Or just use your 5-8th place powerup in 1st or 2nd if no blue shell is coming and it will last you to the end anyway.

Proceed to victory.

Point being: If a gameplay mechanics are not fair, don't worry about them, just think about how they can be worked to your advantage. 99% of players think of the blue shell, but its a canard. Very few think of the powerup hoisting, but that's whats going to win you the game.

The final act is your friends complaining that you always seem to win and are unaffected by the blue shell. Here again it works for you as a punching bag of plausible deniability. Yes, the unfair and random blue shell is why you always seem to win...