I'm going to go against the flow here - I see NOTHING wrong with practicing in the park.
I wouldn't walk right up behind someone and blast them, but parks are neither churches nor libraries. Children play there, dogs bark. It's an open-air environment and available for use by all. I don't think the sound of bagpipes is so intrinsically offensive that it would render the park useless to anyone else. In fact, if I heard pipes I'd start looking for them and then settle down to listen. Along with fireworks and 75 percent off sales, they are like magnets for me.
Besides which, pipes are really meant to be played in the open air. If you're learning, you should learn what they are supposed to sound like from the start. If you can practice with just the chanter, of course, that's great.