Although it might seem that the central theme is about the need to focus on darker feelings, like irony, rather than on sentimental affection, the poem's real theme is the necessity—or maybe the urgency—of enjoying what you have while you can. The speaker reminisces about going to antique fairs with his daughter when she was 12 and rarely turning around to look at her. But then, when his daughter is too old to want to come with him, he can't believe that he ignored her just to look at "beautiful things." The irony in the poem's title is about how beautiful his daughter was, how beautiful it was just to be with her, and how he feels like he wasted the moments when he wasn't looking at her by looking at other supposedly "beautiful things" instead of her.
Another theme of the poem is how the past affects the present—he might not be as sad that his daughter is now too old to spend as much time with him if it weren't for his memories of what they used to do together. His happy past now seems painful, as though it is reproaching him for not appreciating the time that he had with his daughter then.