From the point of view of USA's allies in Europe and Asia, Trump has made things very uncertain. Withdrawing from the TPP, claiming that allies should pay for US protection, foreign policy through Twitter, etc., these do not help matters with USA's allies.
With Obama there were some things which signaled low credibility (e.g. talking tough about a pivot to Asia and then not following through when his bluff was called), but at least he doesn't act in a personally whimsical/unreliable manner, and he remained (on the outside) committed to the old US foreign policy of maintaining US hegemony. I don't think Obama can be blamed for not following through on some things: the world is changing and some things just aren't possible any more, for example the US-China status quo balance of power is bound to shift in China's favour unless China implodes somehow. Can you blame the man for being unable to maintain the US-dominance status quo against a country with >1bn population that more-than-doubled its economic output over his fucking _term_ as President?
Trump seems to signal a big change from "US hegemony even at cost to the US" to "we will only do things that benefit us". The latter is perfectly OK: every other country in the world does it, but the USA is a self-appointed world hegemon and this was what other countries' relied upon in their interactions with the US. Now they can no longer rely on this, so any claim to hegemony that is not backed by self-interest now becomes incredible under Trump.
You can't imagine how astonishing and unnatural it is that most other countries in the world accept the US' "charitable offering of hegemonic peace-keeping". This was the kind of credibility that Obama was trying to prop up: the credibility of a world power which is willing to honour its treaties and benefit its allies (think of the Marshall Plan post WW2) even at significant cost to itself. Trump does not and cannot offer this kind of credibility. He offers the credibility that China offers, the sort of business-oriented "let's make a deal that we can both benefit from" credibility.