No, I'm not underestimating him. By all accounts he's very impulsive. Why do you think he never practiced before any of the debates? His temper is also too short by half - and that, along with his demonstrated tendency to publicly make wild assumptions, makes him easily manipulated. The other nations will eat him - and by extension, us - alive.
And when it comes to China, that's why Obama's pushing the TPP...because that particular free-trade deal is America's opportunity to set the narrative on international trade on the Pacific Rim...and if it doesn't pass,
then China will step in. Also, we're not exactly "rolling over" when it comes to China. We're continually transiting our Navy through the waters the Chinese are claiming (and daring them to do something about it), our military in the area has been bumped up significantly (especially in one particular way that I'm not at liberty to say, but I saw it with my own eyes - I may be retired, but my oaths hold true), and China knows beyond a shadow of a doubt that we would go to war to defend S. Korea, Japan, the Philippines, Taiwan, and (probably) Vietnam.
No, China doesn't laugh at Obama. But they would laugh at Trump, for they know that all it would take is a few false-flag attacks of the right type in the right area to get us heavily involved in someplace far away from China's sphere of influence.