Perhaps "stodgy" would be a word that works for you/is what you're thinking of when you're considering stuffy
(of food) heavy, filling, and high in carbohydrates.
"he loves stodgy puddings"
synonyms: indigestible, starchy, filling, heavy, solid, substantial, lumpy, leaden
"rich, stodgy puddings"
However, the other answers are guiding you towards using a positive descriptor and that's the approach I'd take. It would be better to say a smoothie is "light and easily digested", rather than "not too stodgy", because it's better to use a "positive" than use a "not negative" even if they usually are interpreted to mean the same thing. Politicians tend to use not-negatives.. they will say "well, you're NOT WRONG", which can be interpreted to be saying "you're RIGHT", but actually still leaves them some room to say "but you're not right either"..
I agree with the others re stuffy; "stuffy" is generally used to describe an atmosphere that seems hard to breathe, like in a party where there are no open windows. It's probably due to elevated levels of carbon dioxide; humans are quite good at detecting elevated levels of CO2 and perceiving the air as being "not very fresh". It may also be accompanied by high humidity and or stale smells