Can we say 'chubby' about adult people? Would it have a positive or negative meaning?
Yes, you can say
chubby about a person of any age, though it is more frequently used of younger people.
It is mildly rude, but less so than
fat, and it is much friendlier than
obese which is a much more clinical expression.
Your sub-question about whether it is positive or negative is not really relevant - it is a description, and as such can be used in either positive or negative contexts - it is the intent of the speaker that matters, not the word itself. For example, (particularly in areas where poverty is common), describing a baby as
chubby would be a good thing - the obvious comparison is with
skinny which would be a sign of malnourishment. In an adult, in a rich country, quite the opposite is true.
There is no law about using the word chubby. It means "plump and rounded". But if you need to ask about a word, then you know that you probably shouldn't use it about a person.
Calling someone "chubby" would be insulting. As a general rule, don't comment on other people's weight.
Most people would not like to be called chubby.