Is Angra Mainyu really that bad?

No,he's not. You need to have experiences to know how to use him effectively. And he's also really hard to summon. Even if u dont have any gut CE, u will get his bond CE later although it takes quite some times to get.He's an avenger class,O cost servant with class advantage in almost all of the class. There's only 6 avenger class servants, 3 on NA server, 6 on JP. Try testing him on easy quests, if u dont know how to, just ditch him for now. Use some easy 3* servants for now and get levels, new servants and learn the game.
If you are an otaku or watch anime, search on youtube "Nana tsu no taizai" in which the main hero use his ultimate "full counter" which is the exact same skill of Angra Mainyu.

Angra really is that bad, sadly, though it does make sense. He's the weakest heroic spirit, and it shows. Not to say he's useless, but outside of self imposed challenges and a few spoiler battles, he's just bad. His stats are terrible, his skills are pretty bad, and his Noble Phantasm is borderline unusable.

Eventually, he gains an Interlude that makes his third skill (His only good skill) even better, but even with it, he'll be outdamaged by just about everything that hits neutral. If you get him to bond 10, he gets the incredibly niche ability to deal double damage to exactly two enemies, but even at double damage, he'll still probably be dealing less then any three star berserker.

Overall, he's a status symbol more than anything else. Using him in battle isn't recommended unless you're a veteran trying to challenge yourself or just really like the character. It's a shame, because I love his design, but he's got basically nothing going for him except that he's rarer than anything else in the game.

The first thing you need to know is that he basically has the stat of a 2 star, so probably not gonna do a lot of damage to begin with.
Second, his NP doesn't do damage directly, it only hit back if he got hit, so it is difficult to use.
I don't have him but I have seen people make it work, but you have to build a team just for him, and is probably better to get his bond CE.
All in all, it's just a lot of resources to get him to work.

Angra is so bad that he tells you he's the weakest and to not use him in his Bond 1 line. In his Bond 2 he actually tells you to roll for someone Else :V

His kit is basically just a mess - 2 fairly strong defensive skills followed up by a skill that kills him doesn't make the most sense and an NP whose power is reduced by those same defensive skills makes for a very unfortunate combination. The one nice thing is his defensive skills are actually fairly strong(30% attack down, 50% crit down, guaranteed charge drop) so he can do okay on a stall based arts team though until he gets his interlude that leaves him as having just two skills which is obviously not a great thing.