I think I've mentioned before that Sheldon is the "monster" in this situation comedy, the main source/cause of the situations in which the other characters find themselves. One could say that Sheldon
is the situation.
I doubt he's meant to be likeable; off hand, I can't recall him being anything other than the centre of his own universe. But being likeable isn't his purpose, which is to be a substitute for deus ex machine sources of comedic situations.
Such "monsters" don't
have to be unlikeable, but I think it helps that they are, because there needs to be a reason why they are always screwing things up for others; one popular alternative - that the situation character is more than a bit dim (as in
Some Mothers Do 'Ave 'Em**) - risks the audience laughing at someone innocently inadequate rather than someone who, as in Sheldon's case, thoroughly deserves being laughed at because he's intelligent enough to change if he wanted to. (He just doesn't see the need to.)
It's probably best to think of Sheldon as TBBT's version of
Basil Fawlty, or the equivalent of one or other Steptoe (whether it was the father or the son varied depending on the episode) in
Steptoe & Son. That he doesn't change (indeed can't bring himself to change***, as shown in the episode,
The Closure Alternative), even when other characters do, is rather the point of the exercise.
** - Which only got away with it because of all the stunts; otherwise we're just laughing at someone stupid doing stupid things, which I don't think we ought to be encouraging, really.
*** - Either that, or he really is the prisoner of his mental condition, which again risks us laughing at someone who could be seen as mentally ill (at least to some degree).