New MTB tyre looks wobbly
Let all the air out and work your hands around the tire, "breaking" the tire edge from the rim where it has stuck.  Go around twice doing this.  Then reinflate slowly, checking every few pumps to see if the tire is properly centered.  If not, work it back to the center with your hands.  

Pay particular attention to the part of the tire right next to the rim -- there's generally a sort of ridge in the tire sidewall that, if the tire is properly seated, will be a constant 2mm or so from the rim edge.

If you cannot get the tire to seat, regardless, then you may have the misfortune of having mixed fractional and decimal tire sizes.  But first make sure the tube is not twisted inside the tire.