タイトル通り。

結論としては使えます。

※Laravel：6.0.3

サンプル

class User extends Model
{
    protected $fillable = [
        'name',
        'age'
    ]
}

>>> App\User::create(["name" => "hoge", "age" => 99])
=> App\User {
     id: 1,
     name: "hoge",
     age: 99,
     updated_at: "2019-10-09 10:20:33",
     created_at: "2019-10-09 10:20:33",
   }

ミューテタを追加

class User extends Model
{
    protected $fillable = [
        'name',
        'age',
        'birthday'
    ]

    public function setBirthdayAttribute($value)                                                                                      
    {                                                                                                                                 
        $this->attributes['age'] = CarbonImmutable::parse($value)->age;                                                      
    } 
}

birthdayの方で作ってみる

>>> App\User::create(["name" => "fuga", "birthday" => "2000-04-01"])
=> App\User {
     id: 2,
     name: "fuga",
     age: 19,
     updated_at: "2019-10-09 10:35:48",
     created_at: "2019-10-09 10:35:48",
   }

fillableに残っているのでageもそのまま使える

>>> App\User::create(["name" => "piyo", "age" => 50])
=> App\User {
     id: 3,
     name: "piyo",
     age: 50,
     updated_at: "2019-10-09 10:36:01",
     created_at: "2019-10-09 10:36:01",
   }

birthdayで必ず作りたいならfillableからageを外してしまえばokです。