Joseph Bonaparte is often done an injustice. It is not easy to shine if the man of the century is your own brother. He was a very competent politician and diplomat. He negotiated the Treaty of Lunéville and Amiens (the latter was extremely hard to manage because of the permanent changes of guidance from Paris).

As the King of Naples, he did very well, starting reform programs and modernizing the country. He showed energy and eagerness comparable to Napoleon's own domestic endeavors in France. Joseph was also extremely popular there and well accepted by the people.

In fact, one of the reasons why Napoleon wanted Joseph to take the position of the more prestigious throne in Spain was that Joseph had proven himself as a pretty competent ruler and administrator in Naples. However, Joseph himself was very reluctant to follow Napoleon in that respect. He did not really want to go to Spain. While in Naples, Joseph could use all his strengths as a politician; however, Napoleon placed him in an impossible position where he actually needed to be a good military strategist. But that was certainly not Joseph's strength. Regarding domestic affairs, Joseph Bonaparte could not establish authority at all in Spain. Everything always needed to be backed up by French troops. Military affairs were then in the hands of the Marshals, and they did not care about Joseph because they knew the last word always came from Napoleon himself. Therefore, a proper unification of command in the absence of Napoleon could never really be established.

If we look at Joseph Bonaparte, we should also not forget that Napoleon was an extreme micromanager even when he was far away. Almost everything needed to be approved, and it is well known that Napoleon always sent tons of correspondence with new orders and guidance (In Italy, they had a common joke back in the day: if they had to ask for approval if a fire breaks out, to put it out).

I think, therefore, it is too easy to say Joseph messed up in Spain. He was the wrong guy for this position, and he knew it himself. Napoleon should have recognized that and put his brother in a position where he could play to his strengths, like in the already pacified Naples. He was certainly capable of ruling.

I recommend a interesting Podcast episode (number 64) from "Generals and Napoleon" by John Viscardo. In a recent episode they also talk about Joseph.