When poco, molto, troppo precede another adjective they function as adverbs.They will also function as adverbs when they modify a verb, e.g., lei mangia poco (“she eats very little”). As all adverbs, they are invariable. E.g.,
Lei è molto intelligente.
She is very intelligente.
Le macchine sono troppo care.
The cars are too expensive.
Loro sono poco pazienti con i clienti.
They are not very patient with their clients.
You could also see poco/molto/troppo in front of another adverb, in this case they would also function as an adverb and never change their form:
Lei non parla molto bene. = She doesn’t speak very well.
Loro non guidano troppo velocemente. = They don’t drive too fast.
Similarly, you could also see poco/molto/troppo in an invariable state after a verb:
Lei mangia poco e parla troppo. = She eats very little and speaks too much.