The question of Ludwig II's mental health is a very grey area with no clear answer. Ludwig II had exhibited a number of erratic and odd behavior which his deposition concluded he suffered from an advanced state of insanity which many have since construed as schizophrenia. The Third Reich would further popularize this alleged madness as the Wittelsbach were not friends of the new regime. However, the lead physician of the deposition, Bernhard von Gudden, never once consulted the actual patient. Der Spiegel's article is correct in that von Gudden's report met in secret and many of its interviews were secret. It goes without saying that a deposition of a monarch is a highly politicized affair and Bismarck warned against a secret deposition that it could look like a palace coup.

Ludwig II's instability and violent outbursts are a matter of record. There were a number eyewitnesses of him dressing as Louis XIV and or boxing the ears of servants that displeased hims. Modern psychiatrists looking at the evidence collected by von Gudden tend not agree with the diagnosis of schizophrenia or extreme delusions. Förstl et. al argues on the basis of Ludwig's autopsy and the deposition evidence that Ludwig may have been schizotypal. Haffner argues that some of his eccentric behavior may have stemmed from brain injuries due to meningitis contracted as a child. Christopher McIntosh, one of Ludwig II's biographers, puts forward the theory that advanced syphilis would have explained his behavior.

You have to keep in mind that these diagnoses are all conjecture and replicate much of the same original sin of von Gudden of relying upon hearsay evidence. Ludwig II towards the end of his life did act in an unusual manner and this behavior alienated a number of people within his household. It is impossible to say whether or not they exaggerated or told the truth during the deposition.

Sources

Förstl, H., G. Immler, M. Seitz, and R. Hacker. "Ludwig II, King of Bavaria: a royal medical history." Acta Psychiatrica Scandinavica 118, no. 6 (2008): 499-502.

McIntosh, Christopher. The Swan King: Ludwig II of Bavaria. London: I.B. Tauris, 2012.

Sommer, F. "War das psychiatrische Gutachten B. von Guddens über König Ludwig II. von Bayern korrekt?." Der Nervenarzt 82, no. 5 (2011): 611-617.