Inheriting a blog with years of accumulated posts means inheriting years of accumulated inconsistency. Some headlines shout in title case, some are lowercase, a few are entirely in capitals from an author who felt strongly.
Why it reads as amateur
No single headline looks wrong. The set does. Scrolling a category page where every third heading follows a different convention signals that nobody is minding the publication, and that impression transfers to the content itself.
Normalising in one pass
Export the headlines, convert to sentence case, and the archive reads as one publication. It is among the cheapest credibility improvements available on an inherited site.
Then fix the proper nouns
Automated conversion will lowercase brand names, product names and acronyms. Scan for them before republishing — iPhone rendered as Iphone across three hundred posts is worse than the inconsistency you set out to fix.
Try it: Convert Text to Sentence Case on SeoWolf's Notepad.