The Tyrells are in a deceptively weak position. They control KL, but not the King, and assuming Cersei is proved innocent in trial by combat, will be embroiled in nasty intrigue and complicit in Cersei's mis-government. Militarily, they're in trouble too: Euron's raids on the Reach were nasty, and their army could well be trapped between the GC holding Storms End, and a Martell army attacking their flank. Also, even if Mace doesn't want to declare for Aegon, many of his bannermen will want to. They fought for Renly against the Lannisters, and have typically been staunch Targaryen loyalists. The only person the Tyrells have enthusiastically followed thus far is Renly (young, charismatic, honourable etc.). They followed Stannis partly out of fear of killer shadows, and partly convenience. Ditto they followed the Lannisters out of expediency. If Aegon becomes a better prospect than Tommen, then they'll stab Cersei in the back immediately.