This is a wild one. Being in a pool could be considered negative I guess as you aren't moving toward your goal of getting those hours toward moving to the majors. If it's a pool before day 1 Indoc then you aren't getting paid, so that's negative. If it's after Indoc, then you're getting paid without having to work, so it could be positive or negative depending on your situation and if you're a half empty or half full kind of person.

The word itself doesn't have any connotation. If you're worried about being considered just a number before you even show up to your first day of work at a large company, I promise that's exactly what you are whether they use the word pool or something else. Depending on the airline, that's probably what you'll be for the most part after that as well. Even the smaller regionals have 500+ pilots. How many of those do you think the CEO could pick out of a line up?

But...no one was trying to insult you by referring to you as being in the pool. It just means you're waiting for an opening in the next phase of training.