To say something is a 見物, does it have a derogatory nuance like we are making fun of that person / that thing?
If so, is it derogatory to the extent that even if I intended it as a fun joke it seems a bit too much, or is it just a bit derogatory and fine to use in informal situations?
It can be. The most common usage of this term is as follows:
A) Hey, I'm going skiing with Jack, together with his brand new girlfriend.
今度ジャックと、ジャックの新カノとスキーに行くんだよ。
B) Why? He has never been skiing and he sucks at sport.
なんでスキーなの？あいつスキーやったことないし運動おんちじゃん。
A) Yeah! It will be a 見物! hahaha
だから見物なんじゃんｗ
When you say 見物, it implies that you are looking forward and will enjoy seeing how he/she will fail. So you risk to be seen as an asshole. If you are really close with the conversation partner and you fully understand the nuance of this term, you could use it to joke, but my advice is to stay away unless you are really confident.
Usage of 見物 as simply something worth seeing is rather archaic now and is not commonly used.
I don't think it's derogatory per se. My dictionary defines it as "a sight/attraction/something worth seeing".
But if used sarcastically I guess it would be.
For everything in general, it's not just what you say, but how you say it.
