Skill yes, Intent probably not.
While it takes a fair bit of magical prowess, by the end of year 6 we can be reasonably sure Malfoy is skilled enough that he could have killed Dumbledore.
Malfoy was shown to be similarly skilled as Harry when it came to curses and jinxes during their multiple engagements, while Crabbe and Goyle are shown to be complete lack-wits possessing little to no skill. Yet just the next year they are shown to be able to cast Avada Kedavra easily.
Crabbe wheeled round and screamed, ‘Avada Kedavra!’ again.
Ron leapt out of sight to avoid the jet of green light. The wandless
Malfoy cowered behind a three-legged wardrobe as Hermione
charged towards them, hitting Goyle with a Stunning Spell as she
Malfoy also was shown to be able to handle the imperius curse earlier that year which is part of the trio of unforgivable curses.
Another note is that intent is most likely required, as it is for Crucio and imperius.
And another memory darted through his mind, of the real
Bellatrix Lestrange shrieking at him when he had first tried to use
an Unforgivable Curse: ‘You need to mean them, Potter!’
As we saw in the book, in the end Malfoy did not kill Dumbledore because he didn't really want to, despite being pressured by the other Death Eaters and knowing his own life was at stake should he fail.
‘We’ve got a problem, Snape,’ said the lumpy Amycus,
whose eyes and wand were fixed alike upon Dumbledore, ‘the
boy doesn’t seem able –’
TLDR; Malfoy's most likely able to cast a working killing curse in general. At the scene on the tower, Malfoy's Curse may or may not have worked due to his extreme hesitation and apparent lack of desire to kill Dumbledore.