Well it depends what Stats your investing your EV's into. But to put it simply Relaxed isn't the worst nature your Beldum could have. I consider that you'll be evolving this into a Metang and then into a Metagross correct?
Well Relaxed Nature is + Defence - Speed, with that being said you might think well that's not very good but the thing is that the majority of people who use Metagross in competitive battles don't use him as a Sweeper because of his low 70 Base Speed Stat. Metagross is normally used as a Physical Tank that has EV investment in Attack and either HP or Defence.
Adamant Nature is generally the best nature to give a Metagross as with full EV investment and with 31 IV's in it's Attack Stat will maximise it's Attack. However Relaxed isn't too bad of a Nature because if your using your Metagross as a Physical Tank it will boost it's Defence Stat making it more Defensive.
My conclusion is that it is not the best Nature it could of got but neither is it the worst. There are far worse Natures it could of got such as Modest, Timid or Calm (All of which decreases the Attack Stat).
Hope I helped :)
