How are nature and imagination expressed (thematically) in “Fra Lippo Lippi”?

"Fra Lippo Lippi" is a dramatic monologue in which Browning expresses his own aesthetic credo and his ideas about the subjects upon which an artist can best focus in order to achieve greatness.
The setting—Florence in the 1400s—is one in which the painter, whose actual name was Filippo Lippi, is stopped in the street at night by a gang of men who are apparently "going the rounds" and believe him to have committed some misdeed. He then proceeds to give them a brief history of his life and art. Lippi is an orphan who was taken into the care of monks. His talent for drawing has been discovered, but he comes into conflict with those around him because of the way he ultimately wishes to paint. Lippi intends his art to deal with more than spiritual matters. He is told by the monks that

Your business is not to catch men with show,
With homage to the perishable clay,
But lift them over it, ignore it all,
Make them forget there's such a thing as flesh.
Your business is to paint the souls of men—

In Browning's view, which he has Lippi express, it is precisely the natural world, which includes "the flesh," that should fire the artist's imagination. The real world—as opposed to the sterile world to which the monks wish Lippi would restrict himself—consists of

The beauty and the wonder and the power,
The shape of things, their colours, lights and shades,
Changes, surprises—and God made it all!
......yonder river's line,
The mountain round it and the sky above,
Much more the figures of man, woman, child,
These are the frame to? What's it all about?
To be passed over, despised? Or dwelt upon,
Wondered at?

The theme is that the physical world, "nature," is as much a part of the spiritual world as the rarefied and non-earthly images the religious authorities want Lippi to confine himself to.
Browning's theme in another monologue, "Andrea del Sarto," is similar. Andrea is considered the "perfect" painter, but he realizes that the artist who incorporates the imperfections of the real world in his paintings achieves a higher kind of art. The falseness of expecting perfection or completion is commented on in the lines,

Ah, but a man's reach should exceed his grasp,
Or what's a heaven for?
Similarly in the famous opening of "Rabbi Ben Ezra":
Grow old along with me!
The best is yet to be . . .

Browning is expressing the view that the struggles of life and of the aging process are to be welcomed, not rejected as signs of the "imperfection" of God's plan.
The basic view of nature and imagination in "Fra Lippo Lippi" and Browning's other works is an optimistic one. Browning sees life, including its negative and seemingly non-spiritual earthly aspects, as something to be celebrated, not despised in the tradition of the devout.