If Professor Charles Xavier is "the planet's most powerful mutant" he does not cease to be "the planet's most powerful mutant" when he is in the Shi'ar Galaxy, as there is still no mutant on that planet more powerful than he.
Prof X isnt the planets most powerful mutant so you should stop carrying on with that point. There is no mention of a specific time in the statement "most powerful mutant on earth" so in this case it is understood that residence is the determing factor for whether or not one qualifies. If it was limited to a certain time period, then their location during that time period would be the determining factor for whether or not they qualify.
If Bill Gates is "the wealthiest man in the United States of America" he does not cease to be "the wealthiest man in the United States of America" when he is in another country, as there is still no man in the United States of America wealthier than he.
If somebody mentioned that Bill Gates was the wealthiest man in America, it is understood that this is qualified by the fact that he lives there. They are not (or certainly are not necessarily) taking into account whether or not he is in the country at that moment. However, if Bill Gates is in another country, at that moment he would no longer be the wealthiest man in the United states (at that location).
The same concept would apply to Marvel earth. Anybody who resides on earth is in contention because there is no specific time given to determine whether or not they were on earth. If there was and they were to leave, for that period of time they would no longer be in contention.