Is using “endorse” snubbish or insulting or belittling, when I say

  1. Thank you for endorsing my comment.
  2. I can’t endorse your comment.

Which one is snubbish (if any)?

It is not particularly any of those things. It is really context dependent and if used in speaking, depends on the way the word is said.

I see nothing wrong with the usage.

‘Endorse’ in the sense of validate or support is quite an innocuous word. Did you mean ‘snobbish’? If you did, that certainly wouln’t apply on the face of it to your examples.

Yes Sir, I meant snobbish. Years of reliance on computer word processors have deteriorated my spelling.

Thank you for the answer.