Stats when it's dropped to LVL 50 and Mega-Evolved are as follows, I suppose you can tell me how good or bad it is:
HP- 142
Att- 97 (irrelevant since I have no physical damage attacks)
Def- 102
Sp. Att- 187 (seems to be around the usual?)
Sp. Def- 158
Speed- 117 (probably could be better as far as I can tell but not terrible)
The thing about the Maison is that every stat is
crucial. You're not just trying to get a few wins here and there, you're trying to win every single battle in a row, at least 50 if you're just trying to get a trophy, even more if you're trying to get a good streak.
These are the stats for a flawless Mega Gardevoir I got at level 50: 143 / 105 / 85 / 185 / 155 / 120 with a neutral nature.
As you can see, you obviously have some imperfect IVs and/or some wasted EVs. Your Sp Atk is hardly invested in at all, while you have a lot of wasted EVs in Defense. If your Gardevoir has flawless IVs, it's barely EV-trained at all.
If your Gardevoir is bred flawless, I suggest you re-EV-train it. If it isn't, you should breed a better one and EV-train it properly. And just in case you need to know,
here is cant say's post on IVs and EVs. Your should probably train your Gardevoir to be 252 Sp Atk and 252 Speed (put the 4 in either Def or Sp Def, it's mostly just personal preference here, although you can run some calcs to see which is better maybe?), since I think you're using a full offensive set.
Anyway, good luck! If you need help something else, feel free to PM me :)