This word is acceptable for play in the US & UK dictionaries that are being used in the following games:

• Lexulous
• Wordscraper
• Wordosaur

adj. lacking the ability to impress, inability to produce an impression.

adj. not capable of impressing

"In between, there's a confused tale and a couple of sequences filmed in unimpressive 3D."

unimposing   

Progressive    aggressive    depressive    excessive    expressive    impressive    obsessive    oppressive    possessive    progressive   

unremarkable    uninformative    uninviting    uneventful    unimposing    unresponsive    lackluster    unattractive    unheroic    low-key