Trump's message is fine. It might be a little over-the-top for a sitting President, but for a winning candidate, not yet in office, it is fine.
One must keep in mind that the Left is Evil. The latest manifestation of that was the outrageous slander campaign they conducted against Trump during the last month of the campaign. They compared him to Hitler, called him a racist, rapist, misogynist, moron, and worse. Further, none of them has conspicuously backed down. In fact, had Biden actually believed what he said about Trump in the last weeks, he WOULD NOT HAVE welcomed him publicly to the White House, which demonstration did not show him as a graceful loser, but rather a gross hypocrite. Seriously, if Trump intends to destroy American democracy, how could any sitting President welcome him and wish him good luck?