Tag casing doesn't matter. Capitalize or not as you choose.

Does it matter if tags are capitalized?

Capitalization has no effect on SEO results, and your video with a tag of “Wedding Photography” won't appear any higher or lower than one labeled “wedding photography.” Use capitals if you like, such as for names of places or to separate words in a phrase, but they neither help nor hinder your video's SEO.

Why should you use lowercase tags?

Before HTML5 came onto the scene, web professionals used a different markup language called XHTML to build web pages. When you write XHTML, you must write all standard tags in lowercase because XHTML is case sensitive. ... This strict rule was a benefit to many new web developers.