Is incorrect capitalization considered a spelling error?

I can almost guarantee that different people will answer this both "yes" and "no".

As a Wiktionary contributor I've found some people can be quite forceful in their insistence that spelling applies only to letters and not to capitalization, hyphens, apostrophes, or to the characters in Chinese or Japanese for that matter.

On the other hand you will find many many sentences like this if you search:

• Do you spell Southwest with a capital w?
• I spell my God's name with a capital “G”.
• They are beginning to be aesthetic, like the rest of the world, beginning to spell truth with a capital T.
• The Atman of Shankara is here spelled with a capital “A” while that of Buddha is spelled with a small “a.”

So it is clear that many people do use the word “spell” to cover use of capital letters.

Your question was," Is it a spelling mistake?" The answer is definitely no. It is a grammatical error, a capitalization error, but not a spelling error. When you capitalize a word, you don't change its spelling.

Usually spelling refers to putting the right letters in the right order. Other niceties, such as capitalisation, fall under the more general term of orthography, which includes spelling and other aspects of correct writing.

I think it can be categorised differently. If you know it must be capitalised, it might be a simple typo... If you don't know it, then you could consider it an orthographic error.

Orthography has this origin (according to NOAD)

ORIGIN: late Middle English, via Old French and Latin from Greek orthographia, from orthos ‘correct’ + -graphia ‘writing.’

Spelling is a part of Orthography, linguistically speaking, but as you can see here, "other elements of the field of orthography are hyphenation, capitalization, word breaks, emphasis, and punctuation".

I think it's what you were looking for.