She didn't, Yanina obviously improved her game.
Mark has a good point, but so do you: I'm not so sure how despondent she was against Miki, though she was likely baffled she lost that first set to someone serving one shitball after another.
Players of Wickmayer's caliber tend not to get despondent; they get it together. Without seeing the actual match, we can only speculate, but my guess is that Wickmayer pulled herself together, realized she should be getting her own serve in order, and Mirjana probably was the one that got despondent by a service percentage that got even WORSE. This likely distracted her on her return game and, as WTD noted, led to more scatterbrained play in the final two sets. Mirjana raised her serve % to 54% in the last set, but it nabbed her fewer games, so obviously her play went off the rails, and I suspect it was because she was horrified by her inability to serve.
Without seeing the actual match, though ... who knows?
None of these matches are easy on the WTA, anymore, playing anyone in the Top 100-150. It's important to remember that Yanina had a lot of incentive out there, too: she lost in the first round at IW last year.
Miki may have gone off the boil because of her craptastic serve on this day, but her improved ground game was enough to take a set from Wickmayer (soon back in the Top 30) because of it.
Rest assured, she is head and shoulders above where she was at this point last year, when she couldn't get a hard court win against awful players, much less Top 100 toughies. She's playing Top 60 tennis, for the most part. Now she needs that one match against a top star wherein she's got everything working reliably well (cf. the match against Bartoli last year).
She won't leave Indian Wells in the dumps, that is for sure, despite a bizarre performance against Wickmayer. The girls in the Top 30 are there for a reason: they don't scare easily. Mirjana knows that.
She'll just be more eager than ever to shore-up those deficiencies and get to the next level as quickly as possible.
It's hard for us fans, because we see she can be doing this already. Unless she gets a crappy attitude (which I highly doubt) she is not going away. This foray at IW is easily her most impressive display since Wimbledon of last year (after which she pretty much dropped off the map ... DOH! She won't this time. Losing the way she did to Yanina is not anywhere NEAR losing the way she did to Vinci -- such a gruesome anticlimactic fiasco).