It's not terrible, but in response to your phrasing, none of the following questions is entirely unreasonable (although one or two hint that the questioner -- in this case me -- may be being a bit of a pedantic pain in the bum :-) :
What do you mean by "ideal"?
When you say "21", are you referring to the total of the cards in a hand, or the number of players?
Or does "21" refer to the family of card games of which Blackjack is a member?
Why the "mathematically" qualifier? As opposed to what?
And so given that I'd say there is room for improvement, at least in terms of clarity. One alternative that I think makes the above questions redundant might be:
In Blackjack, is there any sense in which a hand of 21 is the ideal?
I'm still using that vague "ideal", but I think my overall form, reducing as it does subtle cognitive dissonance and uncertainty I feel yours had, leaves little or no doubt that "ideal" means something like "that which is most likely to win".
But this is all very nuanced stuff. No one could seriously criticize your version without deserving a gentle slap around the ear-hole.