Unless there is more to the story, the thread title is misleading and OP is simply making way too much of a simple safety check. Crew have a safety obligation to visually confirm that each passenger's seatbelt is buckled. For larger people this may be harder to see. Presumably the crewmember responsible for the task at your seat could not see the buckled belt, so you were asked.

This is not a leisurely task as the departure won't happen until CC confirm to the Captain that their safety checks are complete. A friendly chat at each seat with each passenger would be quite lovely, but not at the expense of a quick hop over to AMS.

Perhaps this could have been stated better, but "do you want an extension belt" does not equate to "you are fat", it simply imples that you may have difficulty buckling the belt. Over time, as the belts fray, they are clipped until they are replaced. That is a non-issue and it is not that you are getting fatter, simply that you may have hit a shorter belt.

If you send in a complaint, you will receive a standard boilerplate response generated by a computer format, roughly the same format as the other CC are trained to give to anything raised just to keep the peace. So, no,don't bother.