Is there a reason why Lock on/ Sheer Cold combo isnt used much in the VGC other than poor sportsman ship?
There's one Pokemon that learns Mind Reader and Sheer Cold (outside of Smeargle) - Articuno. Although I posit that Articuno isn't particularly terrible in VGC (it learns Icy Wind and Tailwind and has STAB Blizzard if you want it), it's far from the best - it's weak to damn near everything (I'm pretty sure tyranitar Rock Slide is a OHKO), takes two turns to execute Mind Reader + Sheer Cold, and needs to live those turns in the first place. If an opponent switches, Articuno is facing a 30% chance of success.
It's not poor sportsmanship to use OHKO moves - I have seen them used on occasion and they are a viable strategy (for example, it can win you something like Cresselia vs. Sheer Cold Suicune that has less HP and is running low on time due to rest stall), especially for Pokemon that really have little else that is good. OHKO moves are fantastic last resorts; if you have to choose between a win by crit or a win by a OHKO move's accuracy, one is better than the other. However, Mind Reader + Sheer Cold is a pretty poor idea for the reasons I described.