6.17: Disclosure

I don't believe any government or its representatives would place the interests of another civilisation above their own. Earth has been left out to dry by the Asgard central authority before and I find it highly unlikely there are no less than ethical influences in their policies.
Thor has gained the ability to manipulate the current SGC command. Their non-aggressive tactics for obtaining technology are also inline with what he wants the SGC to be. He has vested interests in maintaining the status quo hence is far from an impartial witness for the best interests of Earth. The appointment of the SGC command is none of the Asgard's business yet Thor involves himself, disrespecting Earth's sovereignty and obtaining the outcome that suits him best.
In the Stargate series, this is usually done in the spirit of "our species is more advanced than yours, and we shouldn't help you or give you anything helpful." I still don't see the Asgard as bad as the others Stargate has run in to.
Whatever his motives Thor was an interested party to the briefing and shouldn't have been there.
In sci-fi I like to breath human characteristics into alien civilisations and individuals. IMHO there are no good guys in real world international politics so I reflect this in my interpretations of fictional leaders and representatives.
Undoubtedly there are some members of this forum who would view Thor's appearance as a creatively bankrupt stunt to include the whiter than white good guy everyone knows and loves to bail out the other heroes. Although I don't think I share this perspective towards the writers, at least not yet.

Edit: Even if Thor wanted no involvement in the negotiations and was reluctantly enlisted by Hammond this episode becomes a damning indictment of Hammond. Hammond would have invited a senior member of another government to preside over a highly sensitive meeting over matters internal to Earth. Thor may have had a hidden agenda and the representatives of the other nations had no reason to trust him. If I were representing one of our governments I would certainly not vote in favour of the general after he had involved an unknown outside official in the meeting to try and retain control of the SGC. IMHO not exactly an ace up the sleeve.