N/A - but would most likely be around 50 / 100 assuming an ideal 100-character tier list. Cirno's defenses are bad, and combined with her absolutely horrendous defensive typing which gives her no useful resistances... well, let's just say she was way too overrated in the past. Even with Mountaineer, she is dreadfully frail and moves are few and far between anyways.
Cirno is an ice fairy. While she is described as a weak boss in the Touhou series, if one considers the power of average fairies - who appear often as regular enemies - one can certainly conclude she is exceptionally strong for her kind. Her personality is childish, as one might expect from her appearance, and she is sometimes considered stupid.