I don't see it as sucking up, more as showing that you have taken the time to understand what they are attempting to achieve, and that you are giving thought out feedback:
"I can see you are trying to appeal to teenagers, but the copy comes across as very young."
If I just said, your copy is childish, the recipient would likely think "doesn't he realise I'm trying to appeal to teens? God did he even read the site?"
Without context it's harder to get people to understand and act on your concerns, as most peoples first response is to already be on the defensive and try to find reasons not to implement your feedback.
"I can see you are trying to appeal to teenagers, but the copy comes across as very young."
If I just said, your copy is childish, the recipient would likely think "doesn't he realise I'm trying to appeal to teens? God did he even read the site?"
Without context it's harder to get people to understand and act on your concerns, as most peoples first response is to already be on the defensive and try to find reasons not to implement your feedback.