The word "interesting" can often be used to describe something that wasn't nice.
I know several other answers seem to say that, but, in my opinion, they tend to be describing (slightly, or very) different reasons why.
Let me show you an example of why "interesting" has often become a bit of a negative word.
A parent says to a child: Did you like the coconut cake your grandparent made?
Child's reply: It was interesting.
The reason that answer actually sounds rather bad is because the child could have said:
I really liked it a lot! I thought cakes always had a very sugary flavor. I didn't know that coconuts could be used in cakes. I'd like the recipe so I could know how to make it myself.
(Very positive words are okay to say.)
The child could not have said:
What an awful idea. I hope I never have to put such an awful thing into my mouth ever again!
(Doing so would have been quite rude, and the child should certainly not say that, especially in front of the cook.)
The word "interesting" has become known as "the answer that you can politely give when you really do not want to compliment something because you don't like it, but you want to try to not be rude by saying anything too negative." People have learned that the word "interesting" is often meant to describe something undesirable, but that the person doesn't want to say anything that will sound very impolite when quoted.
By choosing a rather neutral word to describe something that most people have stronger feelings for (like a taste), people often assume an intentional effort to avoid making a stronger answer.
In theory, this technique could actually be used in the opposite way, to avoid complimenting something that is actually good. If I am in a group of people, maybe I don't want to be polite to a friend by not saying something good about a product made by a competitor. So, instead of being rude (to my friend) by saying something nice (about a known competitor) in front of other people, I choose something that sounds rather neutral. In that case, saying something negative might have been more appropriate, but my choice to remain neutral indicated a reluctance to say something negative. Therefore, if I used the word "interesting", that might have been a relatively positive choice compared to what else I might have chosen to say.
That's an unusual example, though, because most people generally like to say very positive things when they have a reason to.
Sometimes, I have used the word "interesting" to describe something that actually fascinated me a bit. That is an entirely appropriate way to use the word. However, when I use the word like this (which is absolutely the most straight-forward meaning), I have sometimes felt the need to clarify: "It was interesting. And, I do mean that. I did really find that to be actually interesting." If I don't clarify, I have sometimes had people ask me for clarification. So, using the word "interesting" to mean "not great" has become quite common.
Side note: the word "different" has also frequently been used in the same way. "That coconut cake was... different." "Unusual" is another word that has been used that way. "Unique" has also been used that way.