The stretching isn't a problem by itself. It's the fact that stretching blurs and pixellates the character. When people refer to "clashing", they're talking about inconsistency. Without the map-tiles looking blurred or pixelly, it makes the two styles look mis-matched (not matching.) the map-tiles look sharp and clear, where the character looks blurry. I like pixellation and blurring doesn't bother me, but if that's you're style, I would suggest blurring (but not necessarily stretching) the map tiles to match it. Another thing I would suggest-- and it's completely your decision to try it-- is this: When stretching the sprite, try to take the top part of the head, and move it down to "squash" it into a rounder shape. This may require more adjustment with some characters than with others--for example, characters with a lot of hair, you may need to do this one small section at a time. Also, try taking the bottom halves of the arms, and move them up a bit, to shorten the arms. It might not be a perfect solution, but by doing this, you may be able to make the sprite tall, but without over-stretching.