While I don't have any problems with Mage Armor, this ties into the value of 1st-level spell slots after awhile. The offensive ones will be laughable to higher hit point foes, the control ones too limited, and the utility ones don't come up often enough. As a Wizard, I've found I have little use for my 1st-level spell slots, so Mage Armor is basically "you get weightless +2 leather armor as a class feature".
Or you get +1 studded leather armor for a price of lv1 slot. Which is not bad for a d6 hp class that tends to be primary target of any inteligent oponent. It's decent, but not OP. How often utility ones come to play depends on campaign and playstyle. 1st level slots at higher levels are usefull for abjuration stuff like protection from g&e, absorb elements, shield and good old silvery barbs
Which, notably is a class feature for Warlocks and one that competes with others (sure, it lets you cast it at will, but really, when is 8 hours not enough?).
Mage armor as invocation is mediocre at best and usefull only in specific scenarios.
So while you're probably not casting it at level 1, by level 5, you have better armor (more or less) than any light armor user has access to without finding magic armor or using a feat. On a class that's supposed to not have armor proficiency as a "balance point" (I find that dubious, and think it's really more for legacy reasons, since getting a high AC in 5e as any class isn't particularly difficult).
Better armor? Yes. Better AC? No. As a wizard, if you optimize build, take ASI, at best you have AC 17 at lv 8 ( dex 18). And that means Int is also 18.
Especially when you have Bladesingers sporting some of the highest AC's available in the game, even in a limited fashion. There's more to defense than high AC, of course, but it does feel strange when the plate-wearing Fighter is outclassed by a guy in a skirt.
Bladesinger,with build as seen above ( dex and int 18 at lv 8) has ac 21 while bladesinging, which he can do in 2-4 econuters per day in tier 1&2 where most games take place. AC is on par with full plate+shield+defensive fs, but he has it all day long. On the other hand, once you get hit, it hurts bad cause of d6 hd and con which is mediocre (14) at best. For 4 rounds in a day, you can raise your ac to 26 with shield spell, and boy you gonna need it if you wanna go head to head in melee with bad guys. You gonna need that haste spell to keep you going (free disengage action to move away).
Guy in a skirt needs that AC for melee build cause he can't take hits.
If anything, 5e shield is underpowered.