It is one of the best ways to get people's attention. Parades that are done for political purposes are eye catching. Whether we agree with the activists points are not. If the people are taking notice, then the politicians better be taking notice. It is also good to see people doing something about what they believe in, instead of just complaining about it.
Yes, I think that parades are an acceptable arena to take a stand on political issues, because parades are a good way for a group to have a non-violent peaceful protest. It is a good way for these people to bring their message to others in a peaceful way, without hurting or inconveniencing others.
Parades are, in general, an outlet to celebrate an event, holiday or otherwise. It is about fun and entertainment, not a place to air out political viewpoints and agendas. There are plenty of other outlets for such things, such as a rally or a town hall meeting. There is also the media, a parade is not the place.
I do not believe a parade is an acceptable arena to take a stand on political issues. If these people want to stand for political issues then they should organize their own marches and protests, not take over community events, such as a parade. I believe Americans need to better assess their decisions and target activities that don't harm the community.