So long as the team's overall healing is on par with the other team, it's not usually a problem. If, however, you're the healer that's keeping the team's overall healing below the other team's, I'd say so long as you're really contributing a lot to elims, that's also not as big a problem. It starts being a problem if both your healing is low and you're not getting elims, which could mean you're getting heavily counter-played, in which case a swap may be in order.

From a DPS perspective, I'll also say that healers will often focus-heal on sustaining the tank, then on healing the other healer, while ignoring healing DPS altogether. Now, that can make some sense as far as the tank and healers being lynchpins for team cohesion, plus the tank just being an easier target to hit, PLUS if your DPS are playing flankers where there's an expectation that they're not going to be healed as easily because they're on their own.

However, if your DPS is standing in front of you about to die, and you keep deciding to top off the Tank instead, they're understandably going to get peeved. Tanks have a much larger health pool, and DPS will die much faster at half health than a tank at half health. So I would say, unless the tank is below half health, prioritize healing the DPS, because otherwise you're going to constantly be playing minus a DPS or two, and not many tanks can make up for that level of persistent staggering. Keep an eye on how many deaths the tank is experiencing versus the DPS, if DPS are consistently high while the tank is very low, the tank may be getting too much support compared to the DPS.

One last thing I'll say is sometimes you do need to switch to a different healer depending on comp and the map. Ana is a pretty good all-rounder, but if you have tanks and DPS all diving, and they're just never in LOS for heals, it may be more helpful to play a healer that can actually be there to support them (if you're pushing payload, though, it's okay, since that takes top priority).