The first line of text will be larger and bold if "New Notes Start With" is set to Title in Settings > Notes.
There is little else you can do. The Notes app does not have selectable fonts. You can type in bold, underline, and italic, but the text should be normal by default.
There are some global settings. The most likely to be turned on would be the Settings > Display & Brightness > Bold Text toggle. This makes text bolder throughout the user interface, in most of Apples first party apps, and in many third party apps. The Text Size option in the same area can have similar effects.
Those controls are duplicated in Settings > Accessibility > Display & Text Size, along with some other options. Again, these are global settings, not specific to Notes.