HELLO RPG WORLD!

The Mario Bros program is better for making Mario games, yes. However, general 2D platformers... probably GameMaker is your best bet.

Welcome to RMN!

game maker is huge fun to use and even people who work in things like unity most of the time hold it up as a good prototyping tool

construct and stencyl are also quite good for that kind of thing. the choice more or less comes down to what you prefer to work in. game maker has a proprietary scripting language, stencyl works in actionscript (or haxe, if you have the beta version), and construct is just editor all the way (but still very versatile; this is being developed in it, for instance)

honestly, since a side-scrolling platformer is basically the archetypal indie game genre, most engines targeted at indie developers will be highly capable of making one. my advice is to try everything for a couple of days each, and then make your decision once you've got some idea of how each of them functions.

e: avoid Indie Game Maker, it's awful and enterbrain might try to make another one if we're not careful