The speaker admits to a kind of lowered standard: joy isn't picky, and will happily take whatever is on offer, even a common crow. That's a funny, slightly rueful idea, since crows are usually thought of as ordinary or gloomy rather than as bringers of happiness. But the speaker isn't sure the crow was ever really seen. "Which I think / I never did see" leaves the whole moment in doubt.
The last lines explain why. The speaker gives their "weak eyes" plenty of freedom, which suggests they let their poor sight wander, guess, and fill in blanks. The title, "Missing the small picture," fits this: details slip past, and what's left is a blurry impression that joy grabs onto anyway.
The mood is wry, modest, and a little self-mocking. There's some sadness in not seeing clearly, but also a gentle humor. The speaker seems to say that joy doesn't need sharp focus, and that a half-seen crow might be enough.