I've had this issue come up multiple times during my time here: a new user comes in full of enthusiasm and things to contribute but their posts are riddled with grammar/punctuation errors and the format is all over the place. I am comfortable with guiding a user through best practices formatting-wise since there is a bit of an art to doing it effectively on the stack specifically.
However, I have always been hesitant to act to overtly encourage a user to improve their grammar. I know there are many reasons a person might have poor grammar. They could be young and still learning, they could never have learned, or they could even be new to the language entirely (and trust me, I sympathise with anybody trying to learn English as a second language). It is entirely possible that a user might even be unaware that their grammar is poor/hard to understand.
However, often the user's grammar is clearly affecting what may be good content in a way that makes it very hard to tell what they are saying (and acquiring downvotes as a result). When this happens repeatedly, I feel that maybe a nudge to tell them that they should pay closer attention to their grammar to improve their answers, but I'm never sure how or if I should do this.
Correcting the grammar/punctuation is one way of course, but after many answers it gets to be tiring and sometimes it is even unclear what is being said and thus there is no easy way to clean it up.
Should I worry about this? Is this just something that users are expected to "get" just from being around and getting downvotes? Is there an effective way to do this that minimizes the risk of insult? Should I not even try?