Against Excellence Absolutely. Sometimes the best and meaningful artwork is by those who are untrained, unbound by what should or should not be considered art, and what is 'correct'. I recommend looking at Outsider Art, often these artists have no problem scribbling then spontaneously dumping their art, forgetting about it completely (only for it to be picked up off the floor and gawked at by those who find this behaviour somehow mystical or enviable). It goes to show that the process and feeling that is applied to creating is more important than the end result. Your example of group singing is bang on, it can be a spontaneous act, shoddily done, that moment played with and thoroughly enjoyed, then discarded. Training to master a skill is the opposite, it's a different kind of thing, like an Art, rather than art.
I suppose an Art is something individually manicured and perfected for others to enjoy, rather than a participatory or playful thing. It's rather mechanic, mathematical, and has a particular goal in mind. (I haven't thought too much about the difference between an Art and art, but do with it what you will). Training to 'perfection' can often limit creativity, something that is vital to a community as new problems are always cropping up that rigid traditional solutions can not combat effectively. There can be a thousand carpenters trained to perfection on a traditional type of joint, on a particular type of wood, and they will all do it the exact same way. But what happens if something prevents them from using this particular wood, what if they have to build something that can not involve this particular type of joint? Will they even know what to do? Will they be too scared to try something else?
Creativity can be a skill in itself, and people are often very wary to step into it, to play even. For a lot of people who have not grown up with a creative background they have to learn to allow themselves to be creative, to think creatively. Often education breeds this out of us as children, even art lessons can be quite authoritarian in that you will be given a task such as "make an angel" with rigid steps and a particular colour. I've seen children shouted at or punished for allowing themselves to be creative outside of the restrictions set by the class. Creative thinking can not be taught by rigid lessons, or How To's, and often the teachers holding these lessons have only two ideas of what art is. It's usually Hyperrealism or Photorealism, renaissance painting... Or so called 'abstract'; Picasso or Van Gogh. So in a way 'Excellence' is a product of our very strange society, hell bent on production, worker mentality, and narcissistic individualism. We are given The Greats to look at, the perfectly skilled, but at the same time punished or shamed for not following the rules. There is more to say about this, which I will neglect for now.
Besides how are we to know what is 'correct' when it comes to creativity, this excellence or correctness is a contradiction when applied to creativity. To create one must expand or recycle, a creative mind constantly plays, mashing ideas together, twisting them into new forms, taking what is known and turning it on its head. There is no aim exactly to it, it is play and everyone can partake in play, and nothing is immune to it. Excellence is the opposite of this, it is about reducing and restricting, instead of recycling there is replicating. If everything can be replicated then it can be sold on mass, this is important to the kind of society with live in and not at all helpful.
About philosophy, I also agree. The nature of philosophy is to wonder, question, recycle, expand, it is truly a creative way of thinking. Children as young as five can do this, this us absolutely Not a criticism but a positive. A society that values philosophical thinking, values creativity, and encourages community and individuality entwined. Just as a choir around a campfire has many different voices, all come together for the same reason.