is a gtx 1070 ti overkill for 1440p 75hz?

Depends on what you mean by overkill. Paired with an adequate CPU the 1070 ti would likely be capable of more than 75fps 1440p in many games. So if you had a higher refresh rate panel the 1070 ti would likely be able to make use of it. In my opinion that certainly does not mean the 1070 ti is overkill for 1440p 75hz.
 
Not in the slightest.

On average, figure a 1060-6 is good for most 1080p/60Hz at max. A 1070 is almost all 1080p/60Hz, many 1440p/60Hz or 1080p/144Hz. A 1070ti is 99% of 1080p/60Hz and most 1440p/60Hz or 1080p/144Hz. A 1080 is good for most 1440p/60Hz or 1080p/144Hz, many 1440p/144Hz and some 4k. A 1080ti being good for most 4k.

So 1440p/75Hz fall right in line with what you are looking at. Of course there will be games like minecraft where yes, the game is so graphically simple you could run 1440p/75Hz on a gtx1060-3 easily, but there will be other games like GTA:V or CoD or Battlefield series where high details and massive amounts of physX will stress even a 1070ti at times.