There is nothing wrong with a Halloween tree. In fact, one recommendation is to keep the tree up all year round, and redecorate it according to the season. After New Years, you put up red and white lights and Valentines Day items. Then you put up green lights and shamrocks and leprechauns. Then pastel lights and egg decorations and chicks and rabbits. And for summer, red, white, and blue lights and patriotic ornaments like flags and stars make sense.
Use your imagination, and keep the tree up all year if you feel like it. There are absolutely no hard and fast rules on decorating a tree, and one could even decorate one for a birthday if one felt like it! Hell, if they really wanted to, they could figure out how to decorate a tree with Puketower material! But I don't think too many people would want to look at such an ugly sight, and it would ruin the whole tree until taken down.