The biggest problem for this franchise is that good players prefer less money to go to other teams.
It all comes from the very top. This franchise has one of the poorest images and reputaion.
Players around the league are very aware of the rigid tactics by the Cohan & company when it is time for them to go to "business" side. Case in point is BD, MP, probably AB now, not sure about Monta. I am not sure I can find many franchises who always show it up to their own players (maybe, Hawks, Clips, so the Warriors are in the same league).
Therefore, Mullin is quite limited on his options. I do believe that it was Cohan who actually nixed the KG's deal by refusing to commit to paying up to him. I also believe that Nellie and probably Mullin would probably like to extend Baron for 4 more years and not screw up the very effort they put into creating the identity of this team. The Warrior's employees are sad people, as they are also treated from the strong hand of the organization. It is felt on all the levels there, know thsi for a fact.
Now, Mullin is not the smartest person on earth. And he is probably a below average GM as well, although he is growing. Nellie obviously is basketball mind there, and Mullin pretty much needs to bounce between Nelli's desires and vision and players evaluations and Cohan's rigid "business" side. The signing of Maggette is bad not much because they overpaid him(and they obviously did because, again, players don't come to play here unless they are overpaid), but mainly because he is not that needing piece of the puzzle. They neded PG distributor and a low post presence. Magg kinda duplicates what they already have in Monta and Jax (although I feel that Magg is a little better player than Monta now, primarily because of his improved 3pt shooting and he is bigger). So, basically it is a so-so basketball move predicated by the urgent need to have someone to come to play here. Until Cohan fixes the situation and when the players will actually want to come to play here (this is a very nice area with a great fan base afterall, look at Phoenix: they are in smaller market but players always respect that organization and want to play there), we will always have to overpay for players and the team will be without stars (unless we really struck gold in draft, and a player like AR turns out to be quite special).
So, to sum it up, although I am not the biggest Mullin as a GM fan, he was dealing and wheeling, and he probably made the most out of the limitations of the "losing/laughing stock" franchise image. It is not as terribly looking at the end (this team, although doesn't have an identity now, is certainly deeper than before).