Stuffy doesn't work here.
One way I see it is that the smoothie is itself clogged (doesn't really make sense).
You could call it dense or thick , but I think you might be looking for filling : filling adjective 4.
Food that is filling makes you feel full when you have eaten it. &bullet; Although it is tasty, crab is very filling. (Collins Dictionary)
ell.stackexchange.com/users/36187/Em.
No, that's not a good use of stuffy .
Though Merriam-Webster only lists it as the third definition , I'd say the most common use of "stuffy" in conversational English is to mean, as MW says: oppressive to the breathing
You might say a room that's a bit too hot and very humid is "stuffy".
The word you're looking for is probably filling : (of food or a meal) substantial and satisfying Although dictionary.com lists "filling" under the British Dictionary Definitions heading, as an American English speaker, I'd say it's a pretty common word in the U.S. as well.
It'd be common to hear that a certain dish or meal is "filling" because it seems to literally fill your stomach.
Even if it doesn't actually literally fill your stomach all the way, something that's "filling" makes you feel full.
ell.stackexchange.com/users/55922/cjl750
As the other answers have noted, "stuffy" is not the word to use here.
An alternative to "filling" might be "substantial", which seems to get to the point you were going for with "stuffy".
Indeed, Merriam-Webster has , as its second definition: ample to satisfy and nourish : a substantial meal
ell.stackexchange.com/users/40259/G Tony Jacobs
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
ell.stackexchange.com/users/56750/Caius Jard
I use the word light .
If at all you want to eat, eat light food; we are having a full-course meal very soon.
light food are light in calorie, light on stomach , and digest quickly.
ell.stackexchange.com/users/3187/Maulik V
I only had a cup of smoothie for lunch
so I'd get hungry pretty soon afterwards.
This is because smoothie is not very stuffy.
It digests easily and fast so that my stomach becomes empty very soon.
As stated above, "filling" is the one-for-one replacement here, but another option is the idiom "sticks to your ribs" .
This implies that the food is substantial enough that it stays in your stomach for a long time (sticking to your ribs from the inside), keeping you feeling full for longer.
An example: I was considering getting a grilled chicken salad, but I might go for something that sticks to your ribs more, like a steak.
Additionally, some of your example sounds a little bit odd to a native speaker.
We don't say "a cup of smoothie" and we don't refer to smoothie as an uncountable quantity.
Also, some of your tenses are strange.
I'm not sure if the speaker is speaking about a past event or relating foreknowledge of a future outcome.
Assuming that it's referring to a past event (consuming the smoothie), I would change your example to the following:
I only had a smoothie for lunch
so I'll be getting hungry pretty soon, considering they aren't very filling.
It doesn't stick to your ribs like a burger would.
ell.stackexchange.com/users/49111/Nathan Young
This is because smoothies are not very hearty .
hearty [hahr-tee] adjective – dictionary.com substantial;
abundant; nourishing: a hearty meal.
ell.stackexchange.com/users/5117/Mazura
"Heavy" is like "Filling", but with a more negative connotation, as in "What can I order that's not too heavy?"
"Their food is so heavy and rich, I need a diet the next day.
"
"I avoid a heavy breakfast on race day."
ell.stackexchange.com/users/56832/Robin L.
Scenario:     I only had a cup of smoothie for lunch so I'd get hungry pretty soon  afterwards. This is because smoothie is not very  stuffy . It  digests easily and fast so that my stomach becomes empty very soon.   Question, does the word make sense here? If not, what's the word for a food which does not digest easily and remains longer in your stomach so that you wouldn't feel hungry for a longer period of time?   (Note: "Hard" is not precise)
Use of word "stuffy" to mean "not easily digested" (or word suggestion)
ell.stackexchange.com
<word-usage><word-meaning><word-request>
ell.stackexchange.com/questions/132296
ell.stackexchange.com/users/39781/Will
Heavy, filling, substantial and stodgy can be used. It's preferable to say "light and easily digested" rather than "not too stodgy," because it's preferable to use a "positive" rather than a "not negative," even if they usually mean the same thing.
Perhaps "stodgy" is a word that comes to mind when you think of stuffy (of food) that is heavy, filling, and high in carbohydrates.
indigestible, starchy, filling, heavy, solid, substantial, lumpy, leaden "rich, stodgy puddings" synonyms: indigestible, starchy, filling, heavy, solid, substantial, lumpy, leaden
The other answers, on the other hand, suggest using a positive descriptor, which is the approach I'd take.
It's preferable to say "light and easily digested" rather than "not too stodgy," because it's preferable to use a "positive" rather than a "not negative," even if they usually mean the same thing.