FriedGreenTomatoes2
When the AfD has nearly one quarter of the seats in the Bundestag you can no longer claim that they aren't 'mainstream'.
The Overton window has shifted, and continuing to call them 'far right' is just left-wingers clinging on to a comfort blanket.
Politicians are only 'far anything' if they are distant from wherever the centre happens to be these days.
The Overton window describes which ideas are considered politically acceptable or discussable at a given time. It does not redefine the ideological position of a party.
A party can become more popular without changing where it sits on the political spectrum. If a party that was previously regarded as far-right gains support, that may indicate the Overton window has shifted towards some of its positions. It does not automatically mean the party is no longer far-right.
Otherwise, the terms "far-left" and "far-right" would become meaningless, because they would change every time election results changed. Political scientists generally classify parties according to their policies, ideology, rhetoric, and historical context, not simply by their vote share.
The AfD having a large parliamentary presence tells us that it is electorally significant and increasingly mainstream in the sense of being a major political force. It does not, by itself, settle the separate question of whether its ideology should be classified as far-right.