What is the meaning of "candy striper" in the following paragraph?

Solution 1:

While @pbasdf comes close with their answer, that the terms is used

metaphorically to suggest that Albee's writing is so dark, it makes even Beckett's work seem brightly coloured.

...It is missing the context that the secondary derived meaning of 'candy-striper' is:

One who cheers others up with their youthful exuberance.

This definition is taken from Wiktionary, but is backed up by quotations such as this from Reflections of a Baby Boomer by Janice Hiatt Steil:

One of the duties a Candy Striper can do is to take the "Cheer Cart" to patient's rooms. The "Cheer Cart" is a mobile cart with books, magazines, playing cards, toothpaste and lots of other sundries a patient may like while staying in hospital.

and Coffee Time by L. Shadows:

Candy Stripers cheer up the patients throughout the hospital. We play games, give candy, and sing.

So the writer is saying that, in comparison to Edward Albee, Beckett writes cheering exuberant works.

Solution 2:

"Samuel Beckett gets all the credit for being the darkest playwright — fail, fail again, etc. — but compared to Edward Albee, Beckett’s a candy striper. In Three Tall Women, Albee demonstrates that when it comes to death and dying, he’s as unflinching as the ward’s oldest hand, and a good deal more expressive." (from a review on independent.com)

Given that the quote compares the 'candy striper' (an inexperienced hospital worker) with 'the ward's eldest hand' (an experienced hospital worker), it clearly is using the definition you gave as a metaphor.

Solution 3:

The young volunteer hospital workers to which you refer were so called because of their brightly coloured uniforms (see Wikipedia) which were reminiscent of red and white striped candy canes (see Wikipedia).

It's unclear whether the writer is alluding to the hospital workers' uniform or the candy cane origin of the term, but in either case is using it metaphorically to suggest that Albee's writing is so dark, it makes even Beckett's work seem brightly coloured.