Sorry to close this so fast. My team recently worked on TemplateData and the dialog in VisualEditor, see WMDE-Templates-FocusArea. Let's see:

1. What should be capitalized and what not depends on a lot of things: the author's preferences, the local community's conventions, and most importantly the language.
2. If you want VisualEditor to show capitalized parameter names, all you need to do is to add labels to these parameters in the template's TemplateData JSON. Example:

"params": {
    "alternative logo": {
        "label": "Alternative logo",
        …
    }
}

3. Another option is to actually capitalize the parameters in the template and turn the lowercase variants into aliases, so they continue to work as before.

It might be technically possible to turn this around, enforce capitalization for all parameters without a label and ask users to add a lowercase label in case they want to undo the capitalization. Our team actually discussed this at one point. We dropped the idea for several reasons:

• When looking at the wikitext the parameter names aren't capitalized either. Enforcing capitalization in one place but not the other is inconsistent and potentially confusing for users that use both VisualEditor and wikitext.
• To undo the capitalization the JSON must look like "iOS version": { "label": "iOS version" }. This will certainly cause confusion as it looks like it shouldn't have an effect, but does.
• This should be rarely needed. I mean, if a wiki's convention is to capitalize parameter names, why not do this and actually capitalize them?