Its only greasy if your emulsion breaks, which adding lots of acid will do. There should be no grease, only smooth sauce. But yes, some foods are generally richer and heavy. Carbonara is even more rich and while some people might add a small squeeze of lemon juice it generally isnt made with acid- the black pepper is what contrasts with the richness

If you want more acid I'd recommend adding things on top: some hotsauce or acidic BBQ sauce, some pickled red onions and chorizo (the Mexican kind, no casing), pulled pork bbq with a NC vinegar bbq sauce (vinegar BBQ sauce is the regional variant where I'm from in north Carolina for pulled pork), carnitas, or pico de gallon, or serve with sides or an acidic beverage

But trying to put an acid like vinegar into the sauce directly will break your sauce and you'll get a weird greasy texture, while also tasting wrong. If your cheese tastes too heavy try using sharp cheddar or another sharper more acidic cheese (here in the us cheddar would be the classic choice for Mac and cheese anyway, its flavor works well as the primary flavor in Mac and cheese), and some people use a little bit of dijon mustard because it can compliment the flavor of the cheese and has emulcifiers in it that can help stabilize your sauce. Don't put a ton though, its not supposed to be an acidic food, just a little, like roughly a teaspoon for a large serving at max. You shouldn't be able to distinctly taste any mustard, that would be weird

For anyone who finds the mustard weird take it up with Alton brown

If you have any questions on making Mac and cheese feel free to ask, I'm happy to help :) I used to regularly make it from scratch