I agree that the current inconsistent situation is not ideal, but at the same time I do not see what exactly is proposed.
Yes, question-titles are displayed inconsistently, but what should we do exactly?
It seems to me the suggestion is to switch the display of titles to the serif font used for posts. But then, for example on the activity page, it would be ...
The font used question titles varies among the various stack exchange forums. Math.se has a serif font, as you noted. But for example Academia.se has a sans serif font:
On the other hand, it seems that the items in the right sidebar use the same (sans serif) font everywhere.
Let us turn it around, what does "Question on probability" convey?
The "Question on" is redundant. Everything here ought to be a question. It is not as if we had different types of threads and you had to signal this one is about a question, as opposed to being a review of a book, or announcing a seminar, etc.
Further, the "...