As standalone titles, Final Fantasy Tactics Advance and Final Fantasy Tactics Advance 2 are great portable tactical strategy titles (and their sprites are top-notch, at that). They are rather juvenile, though. Compared to one of the best games of all time -- not even just within its genre -- that is, to the original Final Fantasy Tactics, yes they're inferior.
Often times, when a sequel is still good, but not enough like the original, it evokes resentment from critics as they vent their disappointment or highlight what they see as wasted potential -- much more so than they would have if not for its namesake.
There are countless examples of this phenomenon, from Final Fantasy XII or XIII (depending upon who you ask), to Diablo III. Taken as nothing more than a Diablo-inspired hack-and-slash, Diablo III is a short, but okay game, sort of a B-minus; but as the successor to one of the most fondly remembered, darkly mystic, genre-defining titles of the last couple decades, well, it's a grrrroawl-foam-at-mouth-F-minus-minus... and so on.
In short: Final Fantasy Tactics Advance is not a mediocre title, realistically speaking it's one of the best GBA games around. It's mediocre next to Final Fantasy Tactics, yes, of course, just like nearly every other tactical strategy game ever made is mediocre to what's virtually the best. Nothing can really hold a candle to that game. Regardless of comparisons, though, calling the sequel (in this case) a "horrible title" is just plain hyperbolic.
No as somebody else stated, some people just have lower standards in gaming, Tactics advance and tactics advance 2, when compared to FFT are mediocre at best, and as stand alone titles which they are not, because nobody bought Tactics advance as a stand alone title, it was because of the epic status of FFT, they are mediocre at best also, Ogre Battle: Knight of Lodis is a far superior title than Tactics advance, and both titles are childs of Ogre Battle: Let us cling together, a title that is far superior to FFT, in fact sakaguchi brought Yazmat onboard Squaresoft, because of that title, he wanted a Ogre Battle Tactics set in the FF universe hence "FFT" a water down version of Let us cling together.
Also Diablo III was not Diablo II, because it really was nothing akin to Diablo II, it was a softcore game with diablo lore and prettier graphics (A theme we are all too familiar with nowdays in gaming) aslo titles better than FFTA ? Suikoden Tactics, Super Robot Wars Original Generation 1-2, Fire Emblem, Knights of Lodis, Vanguard bandits, Advance wars 1-2 etc etc.
Now are they fun ? Sure! Are they up to any of those titles ? Not even close, FFTA suffers from the "Lets do it bigger because stuff" you got 979797 items that offer 798794646 abilities, and only a few are even worth your time.
Also Play Eternal Eyes, i never played it, and did not even knew it existed until recently, awesome game. Also check Arc the lad 1-2-3 Awesome story if you follow the 3 games.