Is it unprofessional to write (in a paper) that something is "really cool", for example, when describing a colliding black hole simulation?
In addition to Dave's answer, I'd say that in general, subjectivity is not very professional when writing academic papers. The point is to convince the reader by presenting only objective argument. Somehow, if I read a paper with something like "it's cool, it's amazing, that's the best, etc", then I might think that the author just ran out of objective arguments.
It sounds unprofessional and unscientific. Imagine also your readers in 30 years, what will they think? They may not even take the paper seriously. Indeed, current potential readers may not take the paper seriously.
What would you think if you read an old paper that describes a black hole simulation as "groovy"?