I like to make substitutions so that my baking is less fattening, but is it "dishonest" to bring a healthy version of a popular dish to a potluck? I've done yogurt brownies, pie with Splenda, cakes with applesauce instead of butter, etc. Will people be disappointed to bite into something and realize it's not "the real thing"?
I happen to think that potlucks are the best possible excuse to try out the most fattening, decadent dish that you'd never make for yourself at home because at the most, you'll only eat a little bit of it. I made cheesecake brownies for a potluck recently because I'd been wanting to try out the recipe. My potluck compatriots ate most of them and I still got to try them. It was perfect. I'm all for healthy eating, but up to a point. I think your question is one of degrees as well as moderation, and the right answer depends on what you're trying to accomplish.
If, when you cook for yourself, you want to keep things as low-fat and low-sugar as possible, that's of course your prerogative. The same is true when you have guests to your home for a meal: If you want to make these substitutions, it's your home and your menu and your decision, so you may as well reflect your style. I'm not saying "healthy" as a categorization, though, because some people (including me) would argue that using sugar substitutes doesn't actually make something healthier unless you can't digest sugar. It makes it lower in sugar and higher in something else.
At a potluck, though, bringing a "healthy" version could seem like you're imposing your food values on everyone else. I'd say the same, by the way, about someone who introduces his/her dish as "only local and organic," or someone who tells everyone which kosher certification was on the ingredients in the dish. Imposing your food values isn't necessarily a problem, especially since it happens in the Jewish community all the time, but it is something to keep in mind. I don't think people will be disappointed, but they might wonder why you made that choice.
This is where the question comes in of what you're trying to accomplish. Are you trying to show the potluck masses that it's possible to make a lower fat brownie or a lower sugar pie? Are you making it so that you and others who are watching what they eat in this particular way have a dessert to enjoy at the meal? Are you so afraid of having real sugar in your kitchen that you're not even willing to risk making one dish, lest you eat it all before even arriving at the potluck? If it's about impressing people and doing something novel, then great. If it's about accommodating your food needs like those who go out of their way to accommodate allergy or kashrut needs, I support that, too. But if your interest in avoiding sugar and fat are because of a fear of gaining weight or some other potentially unhealthy compulsion about health, then I think it's worth examining why this has become your "thing," and if modifying your cooking is really the best way to address it.