Can LSD in very small amounts increase mental ability?
The phenomenon you are referring to is called microdosing. It was brand-new in terms of research focus when you initially asked your question, and it is still a fairly novel and little-studied subject.

A lot of interest in this phenomenon stems from anecdotal evidence by microdosing users, who report beneficial effects. This is however not necessarily an indicator of neuroenhancing potential, since psychopharmacological agents are known to modulate subjective evaluations of performance more than constrained objective measures of it (compare tables 3 and 4 in this article, which does not discuss LSD or microdosing per se, but illustrates this psychopharmacological phenomenon).

Anecdotal findings regarding LSD are substantiated in an article, which found that microdosing LSD improved the self-reported mood of participants.
Additionally, this article showed an improvement of vigour (as per the POMS scale) at the highest dose.
This is, however, still not a behavioural effect, as POMS is a psychometric assessment method.
Still, this further substantiates the mood-enhancing effects beyond direct self-reporting.

Another article reported increases in creativity, focus, and well being, though again, it should be noted that all of these metrics were assessed using participant reporting, and not evaluated based on behaviour.

So to conclude, there is to date no convincing evidence that microdosing LSD improves “mental ability”.
There is however a growing amount of evidence that LSD microdosing improves mood, which in and of itself can be a desirable effect, and can further be argued to contribute to improved “ability” in more complex behaviours such as sustained deductive reasoning or social interaction.
Conjecturing from the first cited article on methylphenidate, modafinil, and caffeine, it is conceivable that a similar effect:


  In conjunction with results from neuropsychological testing we conclude that modifying effects of stimulants on complex cognitive tasks may in particular result from more reflective decision making processes. When not under time pressure, such effects may result in enhanced performance.


as the one seen with modafinil and methylphenidate could be present with LSD microdosing.
Appropriate data to address such complex evaluations is however very scant in general, and to date wholly absent for the emerging phenomenon of LSD microdosing.

TL;DR

No but maybe.