The default paste in the topic editor uses the RTF or Rich Text Format. That will always paste RTF copied from the other application and whether the other application supports and puts the RTF format on the clipboard. For example, Internet Explorer puts RTF on clipboard but FireFox doesn’t.
Always use Paste Plain Text when in doubt
If you use “Paste Plain Text” operation, it will paste the text in your document’s default font. But, it won’t preserve rich text attributes like colors, bold, italic, etc. If that is not ok with you, you have to select the entire pasted text and change the font manually.