Yup. They intentionally made the low level game super easy (although maybe not easy enough for the PC to tank multiple yellow and red cons at once). Tank mercs are super overpowered at low levels. Basically, the strat is the same no matter what class up to probably 50-60 (depending on class I suspect). Just sic your tank on them and help out as you can. Past about level 60 or so, the tank merc becomes increasingly less powerful relative to the mobs. If you don't have healing spells, you'll find that there's too much downtime between fights to make it worth using (although using a tank merc and healer merc is still viable in a group that has neither). One way to stretch the tank mercs abilities is to make sure to get GL buffs on him (set merc to passive and it'll only charge you upkeep once, waiving it from then on, so you can sit in the lobby and let the buffs magically appear). Regen is probably the biggest one to get if you can.
The good news is that your own ability to tank will increase as you level as well. You may not want to tank multiple yellows and reds, but you'll find yourself being much more capable of tanking relative to your merc as you gain levels. For awhile (probably 50-80 or so), you'll probably do best with a healer merc. And after that, depending on content, you should be able to either solo directly, or use a caster merc to increase kill rate while you handle the mobs yourself, relying on a healer only when fighting tougher mobs and/or named stuff.
To give you an idea of how "weak" tank mercs get over time, I can't say for sure when this happened, but I noticed sometime in my early/mid 80s that my wizard had higher hps than his J5 tank merc. Obviously can't speak for total AC (mitigation and avoidance is probably not similar), but that by itself still speaks volumes about relative strength of mercs compared to player characters. And that's how it should be. At the low end game, the whole point is to help you just rocket through as quickly as possible (while hopefully learning at least some of the game along the way). At the higher levels, mercs will do "ok" to fill in a spot that's missing, but a real player will always do a better job. Well, assuming they're remotely capable at playing their character that is.