Using skin preview is allowed or no?

Generally speaking, skin programs that a) do not alter the in game environment for you (good or bad), and b) do not give any inherent advantage are allowed. This excludes those which utilize game files - such as those that permit you to "preview" skins from Riot itself.

Further, as another response mentions, even if you do not use the features that provide alteration for the environment/advantages, the program itself may be flagged if it contains such features. Thus, it's better to just play the game, and look at skin spotlight videos and/or gameplay with them to decide if you want them.

No, it's not allowed.

It's not allowed. From what I can gather from Google it allows you to use official Riot skins, which you can only use legally if you purchase them via the in-game store.

No LoL is the only MOBA on the market that doesn't let you preview skins (in-game or otherwise). Because most of them are shit and they like to milk fools.