VSCode user snippet doesn't works inside jsx

Maybe adding "scope" to your snippet:

"scope": "javascript,typescript,javascriptreact",

javascriptreact ---> jsx files

It should be like this...

"JSON stringify": {
  "scope": "javascript,typescript,javascriptreact",
  "prefix": "jst",
  "body": [
    "<pre>{JSON.stringify($1, null, 2)}</pre>"
  ]
},

Putting that snippet into your global snippets file should work.

Gear Icon/User Snippets/ myGlobalSnippets.code-snippets