Can I, when creating a human variant character, choose a +2 ability improvement instead of a feat? In the end I will have a +2 to one ability and two +1 to two other abilities.
No, you cannot choose to get a +2 ability improvement instead of a feat
No rule exists allowing a feat to be replaced with an ability score improvement. Instead, the opposite is true:
At certain levels, your class gives you the Ability Score Improvement feature. Using the optional feats rule, you can forgo taking that feature to take a feat of your choice instead.
Page 165 PHB, Feats
Thus, the class feature Ability Score Improvement can be replaced with a feat and feats cannot be traded for ability score improvements. It should also be noted that this rule specifies that the trade can only be done with the specific feature provided by your class only, making ability score improvements from racial traits ineligible for trade.
Is it legal? No. Is it broken? Probably not.
The rules clearly state that it is possible to choose a feat instead of an ability score increase, when using the optional feat rules. It does not say the opposite is true. Therefore, by a strict reading of the Rules As Written (and by an inferred reading of the Rules As Intended), you cannot exchange a feat for an ability score increase.
That being said, if x=1, then 1=x. The two are clearly meant to be about the same in terms of character advancement, since you are allowed to exchange an ASI for a Feat. Although the rules do not allow for it, it is not likely to seriously unbalance the game if it were to be allowed as a house rule. Discuss it with your DM, if you are a player, or consider it as a possibility if it appeals to you as a DM.
Keep in mind the hard caps to Ability scores that are built into the game. This could allow starting characters to easily start with an 18 in their primary ability, which might not appeal to some tables (though it is certainly possible already when using a rolling system rather than array or point-buy). If this is a concern, but you still like the idea, the ASI-instead-of-Feat rule could be allowed only for abilities which have not already received an adjustment, leading to higher average scores without bumping up the normal maximum.