weather是一个列表：

{
  "weather": [
    {
      "id": 800,
      "main": "Clear",
      "description": "clear sky",
      "icon": "01d"
    }
  ]
}

您必须使用其索引来获取值：

${snapshot.data!.weather[0].icon}

编辑：我发现您的模型没有考虑整个json。下面是完整的模型：

// To parse this JSON data, do
//
//     final weather = weatherFromJson(jsonString);

import 'dart:convert';

class Weather {
    Weather({
        required this.coord,
        required this.weather,
        required this.base,
        required this.main,
        required this.visibility,
        required this.wind,
        required this.clouds,
        required this.dt,
        required this.sys,
        required this.timezone,
        required this.id,
        required this.name,
        required this.cod,
    });

    final Coord coord;
    final List<WeatherElement> weather;
    final String base;
    final Main main;
    final int visibility;
    final Wind wind;
    final Clouds clouds;
    final int dt;
    final Sys sys;
    final int timezone;
    final int id;
    final String name;
    final int cod;

    factory Weather.fromRawJson(String str) => Weather.fromJson(json.decode(str));

    String toRawJson() => json.encode(toJson());

    factory Weather.fromJson(Map<String, dynamic> json) => Weather(
        coord: Coord.fromJson(json["coord"]),
        weather: List<WeatherElement>.from(json["weather"].map((x) => WeatherElement.fromJson(x))),
        base: json["base"],
        main: Main.fromJson(json["main"]),
        visibility: json["visibility"],
        wind: Wind.fromJson(json["wind"]),
        clouds: Clouds.fromJson(json["clouds"]),
        dt: json["dt"],
        sys: Sys.fromJson(json["sys"]),
        timezone: json["timezone"],
        id: json["id"],
        name: json["name"],
        cod: json["cod"],
    );

    Map<String, dynamic> toJson() => {
        "coord": coord.toJson(),
        "weather": List<dynamic>.from(weather.map((x) => x.toJson())),
        "base": base,
        "main": main.toJson(),
        "visibility": visibility,
        "wind": wind.toJson(),
        "clouds": clouds.toJson(),
        "dt": dt,
        "sys": sys.toJson(),
        "timezone": timezone,
        "id": id,
        "name": name,
        "cod": cod,
    };
}

class Clouds {
    Clouds({
        required this.all,
    });

    final int all;

    factory Clouds.fromRawJson(String str) => Clouds.fromJson(json.decode(str));

    String toRawJson() => json.encode(toJson());

    factory Clouds.fromJson(Map<String, dynamic> json) => Clouds(
        all: json["all"],
    );

    Map<String, dynamic> toJson() => {
        "all": all,
    };
}

class Coord {
    Coord({
        required this.lon,
        required this.lat,
    });

    final double lon;
    final double lat;

    factory Coord.fromRawJson(String str) => Coord.fromJson(json.decode(str));

    String toRawJson() => json.encode(toJson());

    factory Coord.fromJson(Map<String, dynamic> json) => Coord(
        lon: json["lon"].toDouble(),
        lat: json["lat"].toDouble(),
    );

    Map<String, dynamic> toJson() => {
        "lon": lon,
        "lat": lat,
    };
}

class Main {
    Main({
        required this.temp,
        required this.feelsLike,
        required this.tempMin,
        required this.tempMax,
        required this.pressure,
        required this.humidity,
    });

    final double temp;
    final double feelsLike;
    final double tempMin;
    final double tempMax;
    final int pressure;
    final int humidity;

    factory Main.fromRawJson(String str) => Main.fromJson(json.decode(str));

    String toRawJson() => json.encode(toJson());

    factory Main.fromJson(Map<String, dynamic> json) => Main(
        temp: json["temp"].toDouble(),
        feelsLike: json["feels_like"].toDouble(),
        tempMin: json["temp_min"].toDouble(),
        tempMax: json["temp_max"].toDouble(),
        pressure: json["pressure"],
        humidity: json["humidity"],
    );

    Map<String, dynamic> toJson() => {
        "temp": temp,
        "feels_like": feelsLike,
        "temp_min": tempMin,
        "temp_max": tempMax,
        "pressure": pressure,
        "humidity": humidity,
    };
}

class Sys {
    Sys({
        required this.type,
        required this.id,
        required this.country,
        required this.sunrise,
        required this.sunset,
    });

    final int type;
    final int id;
    final String country;
    final int sunrise;
    final int sunset;

    factory Sys.fromRawJson(String str) => Sys.fromJson(json.decode(str));

    String toRawJson() => json.encode(toJson());

    factory Sys.fromJson(Map<String, dynamic> json) => Sys(
        type: json["type"],
        id: json["id"],
        country: json["country"],
        sunrise: json["sunrise"],
        sunset: json["sunset"],
    );

    Map<String, dynamic> toJson() => {
        "type": type,
        "id": id,
        "country": country,
        "sunrise": sunrise,
        "sunset": sunset,
    };
}

class WeatherElement {
    WeatherElement({
        required this.id,
        required this.main,
        required this.description,
        required this.icon,
    });

    final int id;
    final String main;
    final String description;
    final String icon;

    factory WeatherElement.fromRawJson(String str) => WeatherElement.fromJson(json.decode(str));

    String toRawJson() => json.encode(toJson());

    factory WeatherElement.fromJson(Map<String, dynamic> json) => WeatherElement(
        id: json["id"],
        main: json["main"],
        description: json["description"],
        icon: json["icon"],
    );

    Map<String, dynamic> toJson() => {
        "id": id,
        "main": main,
        "description": description,
        "icon": icon,
    };
}

class Wind {
    Wind({
        required this.speed,
        required this.deg,
    });

    final double speed;
    final int deg;

    factory Wind.fromRawJson(String str) => Wind.fromJson(json.decode(str));

    String toRawJson() => json.encode(toJson());

    factory Wind.fromJson(Map<String, dynamic> json) => Wind(
        speed: json["speed"].toDouble(),
        deg: json["deg"],
    );

    Map<String, dynamic> toJson() => {
        "speed": speed,
        "deg": deg,
    };
}