Even the sun-clouds this morning cannot manage such skirts.
Nor the woman in the ambulance
Whose red heart blooms through her coat so astoundingly –
A gift, a love gift
The sun shining on the clouds can’t make a red as beautiful as the poppies Plath is looking at. 
 She’s just stunned by these flowers. 
 
 Note the inversion here too – they can’t be ‘sun-clouds’ since clouds in fact obstruct the view of the sun. This, therefore, is an  oxymoron , a contradiction in terms.  The suggestion is that, despite her originality, Plath ‘cannot manage’ to outdo the natural. There’s a suggestion of solar flares, too. 
 An alternative view is that ‘sun-clouds’ is typical Plath-style compression, conflating two related images, the sun being a  noun-modifier  for the clouds. Sunrises and sunsets are enhanced in beauty when partially obscured by clouds; the glow is highlighted by the contours and pattern of clouds. 
 
 Note also the ambiguity of ‘skirts’. In the first line they are a  metaphor  for the sun rays, but in the second line, with the reference to the woman and her coat, they take on a literal meaning.
Poppies in October