Nice! I thought you were using the D-Pad controller, not the Analog stick (Just something I pictured because of people making remote controlled cars interfaced with Wii controllers with that part), but my parents never use that part of their Wii controller, I'd gladly "borrow" it
You're right it's a bit jittery. Wondering if the sample rate is too high, or you would have to program in a smoothing/averaging formula. You don't need it to update every 1/100 of a second. It could grab all the values in, say a 1/8 or a 1/4 second, do average and display that? Although it would be tough to program out large deviances(because it doesn't know what numbers are right and wrong), but averaging would/should really smooth out the large jitter jumps that are out of wack.
It's been many years since i've coded (VB) but you could see about rounding up the numbers that's being recieved. Not sure if how many digits and decimals it's giving you on each axis, but maybe rounding up the data to the next 10, 100, 1000's digit will smooth it out as well. You don't need to be precise to the 10th of a degree, pretty much just to the degree (for display, but no one will be using it for scientific study and need to calculate vehicle angles to precise measurements.
I know some of the clinometers have settable warning tones (some dual with a warning keep and a danger zone), another idea for you if you wish you impliment that.
Keep it up tho, you're really close. I don't think I can say "keep up the good work" enough!