The first question being asked is an opinion question, so feel free to answer it according to your personal thoughts about "On Turning Ten." Personally, I did not (nor currently do I) find this poem funny. I would say that my main emotion while reading the poem is sadness. There are funny moments within the poem, and they are fun and/or funny because they reminded me of fond moments within my own life. The second stanza is specifically what I am referring to. While I don't remember imagining myself as a wizard that makes himself invisible by drinking milk a certain way, I definitely remember having a wonderfully active imagination during those early formative years.
Starting with stanza 3, the poem became sobering. While I am able to think back at my own "loss of childhood," the poem is more saddening to me now that I am parent. I have two kids under the age of ten, and I have one that is now eleven. Something happened when he turned ten. He no longer plays with his trains. He doesn't run around the house with a towel over his shoulders. He doesn't experience pure joy from simply riding his bike anymore. It has been sad to watch this loss of childhood exuberance, and Collins's poem reminded me of it.
Regarding the Bruce Weber portion of the question, Bruce Weber called Collins "the most popular poet in America," and a big reason for that is due to Collins's conversational style mixed with quite a bit of wit. I would agree that those two things make his poems accessible and fun to read. In my opinion, about the only other American poet that could challenge the "popular American poet" crown would be Robert Frost, and Frost is often just as conversational and witty.