I’m a bit wary of the timing of this question since I’ve been receiving messages intent on framing the Princess of Dorne’s choice of Rhaegar for Elia in a negative light or as a character flaw. I have no way to discern if this is an attempt to hold something against the Princess or if it’s a legitimate question asked in good faith. I’m going to hope assume it is the latter because I think this is a conversation worth having.
The answer is yes, there is ableism on the part of the Princess of Dorne in this situation, not necessarily for refusing Tyrion but more for taking it as an insult that he was offered as a match. From a political standpoint, refusing Tyrion might have grounds because 1) Tyrion is a second son and thus wouldn’t be expected to inherit anything, 2) Tyrion is some 16 years younger than Elia which is not ideal for Elia’s childbearing chances and in practice pushes the actualization of the alliance for more than a decade and 3) Tyrion was expected to die soon after his birth so this match was never happening in any scenario. However, not only was that last point intrinsically tied to his dwarfism, the fact that the Princess of Dorne took it as an insult that Tywin offered Tyrion makes it clear that the issue was with his disability.
If Tyrion wasn’t a dwarf, that match wouldn’t have been either made or taken as an insult. But it is considered an insult by both sides because disabled people are seen as lacking in the eyes of Westeros.
The ableism goes both ways in this situation and it drives the reception of both offers – of Elia for Jaime, and of Tyrion for Elia. From the way Tywin instinctively connected Tyrion and Elia based on their disability, it is likely that he regarded the suggestion of Elia as a bride for Jaime as an insult that he then retaliated by offering Tyrion instead (which was ableist towards both Elia and Tyrion).
The Princess of Dorne in turn took that as a slap to the face because dwarf Tyrion was not a worthy match for Elia. Yes, Tywin did mean to insult the Princess, but if she hadn’t consider Tyrion to be inadequate due to his dwarfism, she
wouldn’t have taken the offer as an insult regardless of how Tywin meant it. That’s not to imply that both of their reactions were on the same level btw, because not only was Tywin equally driven by racism, sexism and his issues with ambitious women as he was by ableism, he also actively weaponized his ableism (towards his own son) as an act of retaliation and I firmly believe that he wanted to put both the Princess of Dorne and Elia in their place by his suggestion. While the Princess of Dorne never displayed the active maliciousness with which Tywin wields his prejudices.