Fedor was the best fighter in the world for at least an 8 year period, and for all practical purposes went undefeated over a ten year span.
Every fighter's skill level eventually declines and Fedor is certainly no exception there.
Chuck Liddell will be remembered as the greatest UFC LHW of his era; his string of losses at the end of his career does not detract from this fact.
Michael Jordan's stint with the Wizards was goofy; it does not affect his legacy.
A string of consecutive losses does have a bearing on a fighter's legacy if they occur in a fighter's prime years (late 20's, early 30's for example).
Fedor turns 35 this year, so a decline is not unexpected.