Hmm, I just learned I didn't know the exact definition of ambivalence. From your wiktionary link (which seems to split the difference between the other two):
Simultaneously experiencing or expressing opposing or contradictory feelings, beliefs, or motivations.
Alternately having one opinion or feeling, and then the opposite.
So it sounds like you want a word that describes a thing (the play) as provoking a mixed reaction in a person who observes or considers it (Bob).
If the play is unclear or disorganized, such as having good parts and bad parts, or leaves the plot unresolved in some way, you could say it was confusing. Synonyms include puzzling, complex, perplexing, befuddling, baffling, mystifying and unsettling.
If the play is intended to evoke different emotions, you could call it bittersweet, if the emotions are both painful and pleasant. There may be other words for different compound emotions that I can't think of off the top of my head.
Riffing on your comment, if the play was simply poorly executed at reaching its end goal or telling its supposed moral, you could say it was contradictory. Synonyms with various flavors for this include contrary, inconsistent, inconclusive, off-message (or off-topic), and paradoxical.
If the play specifically bounced around between different extremes or messages, you could say it was vacillating, clashing or wavering.
I'm not sure there is a single specific word that describes a piece of entertainment or media as poor quality without also implying that it was uninteresting. There are plenty of ways to describe the play as neither good nor bad, but it sounds like you're looking for a specific manner of good and bad combined into a single word.