unimposing

/ˌʌnɪmˈpoʊzɪŋ/
adjective
  1. Not impressive or striking in appearance; modest or plain.
    • Despite his unimposing stature, he commanded respect through his knowledge.
    • The restaurant's unimposing exterior hid a beautifully decorated interior.
    • The building was unimposing, a simple brick structure with no decoration.